When we build simulations for ourselves, they're always attempting to approximate reality as closely as possible. The goal is to learn useful things about our own world or society and to try out many forking paths, in a simplified representation of reality.
If we're in a simulation it stands to reason that whoever is running it is somewhat human-like, and exists in a world that has basically the same physical laws ... or at least, similar enough that sociological and technological development would be the same. For instance, the speed of light barrier is pretty damn inconvenient for us but would be great at blocking an arbitrarily large population and state space explosion. And why are these magic physical constants so arbitrary anyway?
If we are in a simulation, and our simulators did want to limit their resource consumption, adding in a few physical laws that are never really a problem in daily life and which block us from colonising the galaxy would be a nice way to do it.