If the universe could be simulated, you could, in theory, run that simulation on a (very large) piece of paper by hand. Running the simulation would then be equivalent to calculating some numbers and writing them down.
Basically, if I wrote down the correct sequence of numbers, I could simulate you. But is calculating those numbers actually necessary? Is it enough to just have the result of that computation without performing the actual steps to obtain it? If yes, then a random number generator would be all that is needed to simulate our (and infinitely many other) universes.
And if that's the case, maybe it's not such a big leap to imagine that the mere existence of the physical laws as an abstract concept is enough to give rise to a simulation?