Why can't airplanes just keep going up? 
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Why do space crafts take off with rockets instead of just ascending like an aircraft until they reach space? 

Ignoring that the engines only work in air, so say its a military jet. Why is it that we don't go into space simply by using airplane technology to go up as much as possible and then switch to a rocket?
For example (although I mean just one craft, not two) in one of the superman films, a space shuttle is on the back of a 747, but instead of just being transported, it launches off the back of the 747. 
Surely this uses less energy, I'm not sure, but it seems logical that a 747 (even with the weight of the shuttle on it) uses less energy to get to its maximum altitude, than a shuttle launching vertically used to get to a 747's max altitude.
What physically prevents this? Is there something about horizontal flight that is unsuitable for space flight?
EDIT: I didn't make it that clear, but if the 747 was modified so it's altitude was greater, a kind of combination of military jet and 747.
 A: Launching a space vehicle from a plane is an example of air launch and it does indeed provide some advantages like initial altitude boost and making greater part of the launch system reusable. Air launch has recently been gaining popularity, see Space Ship One's White Knight carrier plane and recent Paul Allen's plan for a new commercial spaceship.
However, going from this to a single vehicle with dual propulsion system (one for atmospheric flight and one for spaceflight) has very significant disadvantages. One of the greatest challenges when it comes to launching into space is the huge amount of fuel one needs to use for every kilogram of useful mass. The reason for this is that launching into space is not only about leaving the atmosphere (this isn't far at all: just 100km above your head), but it's also about achieving orbital or escape velocity (roughly 7-8km/s and 11.2 km/s respectively in the most common case). Accelerating a vehicle to this velocity requires huge amount of energy and fuel. See Tsiolkovsky rocket equation and this chart.
This challenge is the reason why launchers employ the opposite approach: unlike planes they jettison every heavy piece of equipment that is no longer useful for the remaining flight (e.g. first stages of a multistage rocket).
Carrying the atmospheric propulsion system with you all the way to space (and all the way to orbital or escape velocity) would make each launch extremely expensive if not impossible. 
A: There are good and bad things about flying through the atmosphere:


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*the air produces drag on your aircraft/spacecraft, so you need to supply power to counteract this

*on the other hand, you can slice a wing through this air and produce lift (generally at a lower fuel cost than firing a jet or rocket engine vertically)

*you can use an "airbreathing engine" (such as a piston engine driving a proeller, or a turbojet or turbofan jet engine, etc), which picks its oxidizer up from the atmosphere rather than (like a rocket) having to carry it from the launch site (very expensive in terms of weight, volume)


The complications involved in trying to make an airplane fly very high and very fast (i.e. trying to get into orbit, or a good part of the way there before handing over the job to a rocket):


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*as you climb higher, the air gets thinner

*to get the same amount of lift from thinner air, you need to either use a bigger wing, or fly through it faster

*as the air gets thinner, the power available to you from an airbreathing engine decreases

*as you go faster, reaching supersonic and the high supersonic speeds, the air flow becomes trickier to deal with (more and more drag; aerodynamic heating)

A: I think one reason we haven't seen something like this (in addition to the fact that an air-breathing engine only takes you so high) is that the things that make a good airplane make a poor space-craft and vice-versa.
A good airplane has large wings (compared to body size) with largish engines with big front sections that act as air scoops for the engines. The body is made to be thin and lightweight for good range (but tough enough for speed of sound work). The body is also not airtight, cabin pressure and oxygen levels are maintained by using excess air pulled in from the engines which is cooled and then pumped through the cabin, excess air is bleed out through valves in the body of the plane (http://www.wisegeek.com/what-is-cabin-pressurization.htm).
A spacecraft doesn't need big wings, indeed large wings would be a handicap as it would be very difficult to engineer wings tough enough to stand the craft's top speed through the atmosphere and still be big enough for normal flight. The same goes for 747-style air-breathing engines, it would be very difficult to engineer them to withstand the heat of re-entry (and as mentioned, they would be dead weight for at least 90% of the trip up and down). A spacecraft would have to be air tight and have its own supply of oxygen, which changes the way you build it -- I imagine making it heavier (is the shuttle heavier than an airplane of the same length?).
You might want to look at the challenges faced engineering the closest plane that matches what you are looking for -- the SR-71, wikipedia should give you some insight into how difficult it was to build it (and it still wasn't a real spaceship).
A: Building a craft that could fly atmospherically in the low atmosphere with air breathing engines, then accelerate to escape velocity with rocket engines then use the same aerodynamic shape as a heat shield on re-0entry is technologically a bit tricky.
A project HOTOL was considered but never got beyond sketches.
Launching from a plane makes more sense. In a conventional rocket you need to accelerate all the mass  from 0 speed on the ground. You need big powerful rocket motors to accelerate through the dense lower atmosphere which need lots of fuel, which you also need to accelerate so needing bigger motors.
You need to get out of the dense lower atmosphere as quickly as possible, which is why you go straight up and accelerate as fast as possible, so that needs bigger motors - and so on. That's why rockets typically have multiple stages which you can throw away to lighten the load.
There are a couple of projects that launch rockets above the low atmosphere from planes - Pegasus is the most successful- but these are limited by the size of the carrying aircraft
