Why isn't it $E \approx 27.642 \times mc^2$? Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides? 
Why can so many equations be expressed so neatly with small natural numbers while recycling a relatively small set of physical and mathematical constants?
For example, why is mass–energy equivalence describable by the equation $E = mc^2$ and not something like $E \approx 27.642 \times mc^2$?
Why is time dilation describable by something as neat as $t' = \sqrt{\frac{t}{1 - \frac{v^2}{c^2}}}$ and not something ugly like $t' \approx 672.097 \times 10^{-4} \times \sqrt{\frac{t}{1 - \frac{v^2}{c^2}}}$.
... and so forth.

I'm not well educated on matters of physics and so I feel a bit sheepish asking this. 
Likewise I'm not sure if this is a more philosophical question or one that permits a concrete answer ... or perhaps even the premise of the question itself is flawed ... so I would gratefully consider anything that sheds light on the nature of the question itself as an answer.

EDIT: I just wished to give a little more context as to where I was coming from with this question based on some of the responses:
@Jerry Schirmer comments: 

You do have an ugly factor of $2.997458 \times 10^8 m/s$ in front of everything. You just hide the ugliness by calling this number c.

These are not the types of "ugly constants" I'm talking about in that this number is the speed of light. It is not just some constant needed to balance two side of an equation.
@Carl Witthoft answers: 

It's all in how you define the units ...

Of course this is true, we could in theory hide all sorts of ugly constants by using different units on the right and the left. But as in the case of $E=mc^2$, I am talking about equations where the units on the left are consistent with the units on the right, irrespective of the units used. As I mentioned on a comment there:

$E=mc^2$  could be defined using units like $m$ in imperial stones ($\textsf{S}$), $c$ in cubits/fortnight ($\textsf{CF}^{−1}$) and $E$ in ... umm ... $\textsf{SC}^2\textsf{F}^{−2}$ ... so long as the units are in the same system, we still don't need a fudge factor.

When the units are consistent in this manner, there's no room for hiding fudge constants.
 A: I would not say I know the answer, but in my belief we tend to hide the ugly numbers. 
See Rydberg's equation $$\frac {1}{\lambda} = R (\frac {1}{n_1^2}-\frac {1}{n_2^2}) $$ where $ R $ hides the ugly number $R = 1.0973731568539×10^{-7}\ \mathrm{m}^{-1}$
Similarly, observe Bohr's second postulate $$L = \frac{nh}{2\pi}$$ where $h$ hides the ugly number $h=6.62606957×10^{-34}\ \mathrm{kg}\cdot\mathrm{m}^2\cdot\mathrm{s}^{-1} $
There can be many more examples, but I guess this is enough to make my point!
Note: the numbers you might call ugly others might consider extremely beautiful, as some people might consider Planck's constant as divine beauty!
Addendum: the number of equations with and without such numbers should be considered, I do not think there would be more beautiful relations than the ugly ones! 
Also, you need to start including all numbers even simple natural numbers like 1 and 2 in the list of ugly numbers! Then by that definition even the equation of time dilation has a "1" hiding there!
Added with respect to comment: the numbers you referred to as ugly in your question were uncommon and complicated, I have pretty much found beauty in symmetry both complete and partial, I read somewhere that plants and all are aesthetically pleasing because of partial symmetry! So maybe the simplicity of rational numbers and familiarity with constants makes them less ugly than others.
A: It seems to me that there's two ways of looking at this question, depending on what you view as fundamental. At the end of the day it's all tied up with the surprising power of dimensional analysis. 
In classical dynamics there are 3 independent dimensionful quantities. These are simply mass ($M$), length ($L$) and time ($T$). Once you've chosen a standard unit for each of these quantities then all other dimensionful quantities are uniquely represented by a number and appropriate units. 
For example energy has units $ML^2T^{-2}$. That means that once you've set standard quantities of mass, length and time then you can talk about energy unambiguously. We usually use SI units where energy comes with the unit Joule ($\mathrm J$) and 
$$1\ \mathrm J = 1\ \mathrm{kg\ m^2\ s^{-2}}$$
From this perspective then, it's very puzzling that $E = mc^2$ works out exactly right. Put another way, the quantity $E/mc^2$ is dimensionless - so altering your standard definitions of mass, length and time doesn't affect it! So why on earth should we find that
$$E/mc^2 = 1$$
rather than the less elegant
$$E/mc^2 = 27.1252$$
The answer lies in a more detailed understanding of Einstein's special theory of relativity. Basically what Einstein achieved was reconciling the following ideas


*

*Physics looks the same no matter what speed you are travelling

*The speed of light is a universal constant


Einstein's solution exactly requires that $E/mc^2=1$. In fact you can derive this equation by assuming Lorentz symmetry and Einstein's notion of proper time. There's a couple of good accounts available online - here and here.
But you could possibly have guessed $E=mc^2$ was true, just from your knowledge of dimensions. Think about a decaying nucleus. This loses mass and emits energy in the form of electromagnetic waves. So the three quantities involved are (naively) $E$,$m$ and $c$. 
Dimensional analysis says that they must be related by an equation
$$E/mc^2 = K$$
where $K$ is some dimensionless number. A strong philosophical principle called naturalness says that $K$ must be roughly $1$. Over the years physicists have found that naturalness is an incredibly good guide to our understanding. If formulas are natural, like $E=mc^2$ it's usually a sign that the underlying theory is sound.
This links in nicely with Rob's answer. He mentions a few dimensionless quantities which aren't close to $1$. Some physicists feel that this shows our models are incomplete. There are many proposed solutions that explain these unnatural quantities. Quite a few might be ruled out if the LHC doesn't see any new particles when it turns on again next year. So perhaps by 2016 we'll be abandoning naturalness as a guide!
I mentioned that there was another way to look at the question. Suppose we take our fundamental units to be mass ($M$), energy ($E$) and speed ($V$). Now of course, $E/mc^2$ is no longer a dimensionless quantity. It has units of $EM^{-1}V^{-2}$ and we can adjust our standard measures so that we get exactly
$$E = mc^2$$
This is precisely what Carl and Rijul were talking about. In a world where your units are fundamentally $M$,$E$ and $V$ there's no mystery - the formula is just a useful mnemonic for an experimental fact. 
Let me know if you want any more details!
A: It's a side effect of the unreasonable effectiveness of mathematics.  You are in good company thinking it is a little strange.
Many quantities in physics can be related to each other by a few lines of algebra.  These tend to be the models that we think  of as "pretty."
Terms manipulated by pure algebra tend to pick up integer factors, or factors that are integers raised to integer powers; if only a few algebraic manipulations are involved, the integers and their powers tend to be small ones.
Other quantities may be related by a few lines of calculus.  From calculus you get the transcendental numbers, which can't be related to the integers by solving an algebraic equation.  But there are lots of algebraic transformations you can do to relate one integral to another, and so many of these transcendental numbers can be related to each other by factors of small integers raised to small integer powers.  This is why we spend a lot of time talking about $\pi$, $e$, and sometimes Bernoulli's $\gamma$, but don't really have a whole library of irrational constants for people to memorize.
Most of constants with many significant digits come from unit conversions, and are essentially historical accidents. Carl Witthoft gives the example of $E=mc^2$ having a numerical factor if you want the energy in BTUs.  The BTU is the heat that's needed to raise the temperature of a pound of water by one degree Fahrenheit, so in addition to the entirely historical difference between kilograms and pounds and Rankine and Kelvin it's tied up with the heat capacity of water. It's a great unit if you're designing a boiler!  But it doesn't have any place in the Einstein equation, because $E\propto mc^2$ is a fact of nature that is much simpler and more fundamental than the rotational and vibrational spectrum of the water molecule.
There are several places where there are real, dimensionless constants of nature that, so far as anyone knows, are not small integers and familiar transcendental numbers raised to small integer powers.  The most famous is probably the electromagnetic fine structure constant $\alpha \approx 1/137.06$, defined by the relationship $\alpha \hbar c = e^2/4\pi\epsilon_0$, where this $e$ is the electric charge on a proton.  The fine structure constant is the "strength" of electromagnetism, and the fact that $\alpha\ll1$ is a big part of why we can claim to "understand" quantum electrodynamics.  "Simple" interactions between two charges, like exchanging one photon, contribute to the energy with a factor of $\alpha$ out front, perhaps multiplied by some ratio of small integers raised to small powers. The interaction of exchanging two photons "at once," which makes a "loop" in the Feynman diagram, contributes to the energy with a factor of $\alpha^2$, as do all the other "one-loop" interactions.  Interactions with two "loops" (three photons at once, two photons and a particle-antiparticle fluctuation, etc.) contribute at the scale of $\alpha^3$.  Since $\alpha\approx0.01$, each "order" of interactions contributes roughly two more significant digits to whatever quantity you're calculating.  It's not until sixth- or seventh-order that there begin to be thousands of topologically-allowed Feynman diagrams, contributing so many hundreds of contributions at level of $\alpha^{n}$ that it starts to clobber the calculation at $\alpha^{n-1}$. An entry point to the literature.  
The microscopic theory of the strong force, quantum chromodynamics, is essentially identical to the microscopic theory of electromagnetism, except with eight charged gluons instead of one neutral photon and a different coupling constant $\alpha_s$.  Unfortunately for us, $\alpha_s \approx 1$, so for systems with only light quarks, computing a few "simple" quark-gluon interactions and stopping gives results that are completely unrelated to the strong force that we see.  If there is a heavy quark involved, QCD is again perturbative, but not nearly so successfully as electromagnetism.
There is no theory which explains why $\alpha$ is small (though there have been efforts), and no theory that explains why $\alpha_s$ is large.  It is a mystery.  And it will continue to feel like a mystery until some model is developed where $\alpha$ or $\alpha_s$ can be computed in terms of other constants multiplied by transcendental numbers and  small integers raised to small powers, at which point it will again be a mystery why mathematics is so effective.

A commenter asks


Isn't α already expressible in terms of physical constants or did you mean to say mathematical constants like π or e?

It's certainly true that 
$$ \alpha \equiv \frac{e^2}{4\pi\epsilon_0} \frac1{\hbar c} $$
defines $\alpha$ in terms of other experimentally measured quantities.  However, one of those quantities is not like the other.  To my mind, the dimensionless $\alpha$ is the fundamental constant of electromagnetism; the size of the unit of charge and the polarization of the vacuum are related derived quantities.  Consider the Coulomb force between two unit charges:
$$
F = \frac{e^2}{4\pi\epsilon_0}\frac1{r^2} = \alpha\frac{\hbar c}{r^2}
$$
This is exactly the sort of formulation that badroit was asking about: the force depends on the minimum lump of angular momentum $\hbar$, the characteristic constant of relativity $c$, the distance $r$, and a dimensionless constant for which we have no good explanation.
A: It's all in how you define the units.   $E = mc^2$ in nice MKSA units;  but then change energy into BTUs  and you'll need the ever-lovable "fudge factor" in there.
People spent a lot (well, some) of time developing self-consistent sets of units largely to keep equations simple, tho' as Rijul pointed out,   assigning ugly numbers to known constants hides a lot as well.
A: Nice question!
Most symbols used in physical formulae refer to physical quantities that can be measured. Hence the name quantity. They are measured in units. If the symbols in the formulae stand in a certain relation to each other then so should the measured values. In your example, if the mass is one kilogram (one can measure this), and if this mass (so the value, not the symbol) is multiplied by the speed of light squared (you can measure this speed) then you can calculate the value of the mass' energy. To see if the relation between the symbols conjectured by the formula is correct one can do a measurement of the energy (though making a measurement of the value of the rest energy of some mass is quite difficult). On this basis, you can accept a formula or reject it.
In mathematical physics (where symbols are manipulated all the time), most symbols do not refer to measurable quantities. For example in quantum field theory. Of course, the final result of all this manipulating must refer to measurable quantities (in quantum field theory these quantities are mostly cross-sections of particle reactions and decay rates of particles) to determine if all the manipulating was worthwhile unless you care about totally imaginary situations.
I think it is clear now why physics theorems (formulae) don't have to be accurate always. Only when the relationship between the symbols is confirmed by measurements then this is true.
The formulae are clean, the corresponding relationship between the measured values to which the symbols in the formulae refer will be not. Well, the cleaner the last the more precise the formulae are confirmed.
We can see also that the formulae of physics hold regardless of the units we use. The formulae are objective manipulations of symbols (of course we do this manipulating), while the measuring units are invented by us. You can say that the unit of distance is a parsec or a Planck length. This doesn't change the validity of $E=mc^2$. If we change the unit (measure) of one of the quantities on one side of the mathematical formula (in this case the measure of$c$), the measure of the unit on the other side will change accordingly ($E$ in this case).
