The problem is more serious than that, in that not only is our universe computationally expensive, it is set up in a way such that it would (apparently) have a lot of trouble doing universe simulations. You cannot simulate n+1 arbitrary bits with just n qubits. This means that a simulation computer needs to be at least as effectively large as what it is simulating. You can assume that some aspects are more coarse grained (so you don't do a perfect simulation of most of Earth, just say the few kilometers near the surface that humans and other life are likely to be), but this is still a lot of stuff.