Imagine that there is a person who prefers to measure the amount of money in his bank account with the value $V$. The equation is $V = C\tanh N$, where $N$ is the actual amount of money in dollars. This person will also be confused:
Why is there a limit ($C$) on the amount of money that I can have? Is there any law that says the value of my money, $V$, cannot be more than $C$?
The answer is that he is just using a "wrong" variable to measure his assets. $V$ is not additive --— it is a transform of an additive variable, $N$, which he has to use in order for everything to make sense. And there is no "law of the universe" that limits the value of $V$ --— such a limit is just a product of his own stubbornness.
The same thing applies to measuringmeasure speed --— it is the "wrong" variable to describe the rate of motion; speed is not additive. The "correct" variable is called "rapidity" --— it is additive, and there is no limit on it.