>There is no reason for one to exist so not having one is the obvious case.
It's a usable supposition, sure, and I agree that being asked to prove a negative is silly, but you can't actually be sure that one doesn't exist. It's not the obvious case at all, it's not even all that obvious as a supposition.
What's more, superficially at least, it makes more sense to believe in a supreme, essentially divine creator than it does to believe visibly enormous complexity deriving from a mostly unknown nothing.
I'd say that this more than anything has been responsible for virtually all cultures in history believing in supreme, divine creators of one kind or another vs no historical cultures that I know of believing in the universe springing from random chance and hand-wavey nothingness behind it.
We could also of course be living in a large ice cream, you can't be absolutely sure that's not the case either.
Though, the idea of being the creations of a tremendously powerful and conscious being that created a universe hospitable to our use and for our potential given by all our evident cognitive and material tools seems to me a lot more plausible than being subject to an accidental existence in a gargantuan ice cream environment.