Why can't it be that we simply live in a real universe? That's the simplest answer, the one that requires the fewest assumptions.
The argument goes that: a sufficiently technologically advanced society would run ancestor simulations. Those simulations may also run simulations. There's no ceiling on the number of nesting simulations. It's the height of conceit to think we're the top level when there are squillions of simulated universe.
Right but we don't know what the real universe's limitations are, and I'm geostationary to speak too authoritatively of the capabilities of an arbitrarily advanced civilization.
I don't think simulation theory is true. Eg calculating gravitational forces between everything in the universe would presumably be extraordinarily cost intensive, but essentially irrelevant (I mean like gravitational waves, not the moon).