what makes you think we aren’t living in the simulation?

Since this is the reality we live in it’s a moot point if it’s real-real or simulation-real. It is real real to us even if it’s simulation real to whatever deity resides outside this reality.

That said, most religion claims this is a simulation (this is not the true world and upon your death you will ascend to the world of the Gods, sound familiar?), since humans have felt this inkling for millennia there’s probably some legitimacy to it.