While I agree that simulation theory pushes the question up a level, that is not the case for the God of classical theism.

Ok, I'm curious. How does the explanation for everything's existence being "God did it" not simply push the question up a level to wondering how to explain God's existence?

The general idea is that the set of all contingent things can only be explained by something non-contingent (ie necessary).

A necessary thing, by definition, is its own explanation. "I am what I am" etc.

From there, classical theists attempt to connect that necessary thing to what you'd commonly understand as God.

I just noticed that if you replace "God" with "Big Bang", it's pretty much the same question.

The answer to Big Bang is that before it, there was no time itself, so there's no notion of "before" (or if you wish, it's an error in the question itself that assumes there was "before").