I can’t say for sure that we’re not, but to me it just comes across as an outlandish concept. Much of our natural world, while often bizarre and strange, can be explained through observation and empirical reasoning. When a concept like universal simulations comes around I usually just land on pragmatism and practicalities: for the theory to be true, so many things that are beyond our comprehension would also have to be true to allow it, and since the simplest explanations are usually true, the simple explanation here is that our reality is what it appears to be (with all the cosmological caveats that kind of thinking entails).