Why does anything at all exist?

[Alex K]

Then I could ask why does quantum physics exist instead of nothing.

That's true.

"Why" questions trick you into believing that they are valid questions, just because you can form them in the English language.
What exactly are you asking for when you ask "why"? Are you looking for a cause? Then you are out of luck, since cause and effect are only applicable in the macroscopic realm within our universe. A logical necessity of some sort? There may not be one, see why ontological arguments fail - they might fail for the universe as a whole just as they fail for god figures. Logical conclusions need premises or axioms, and you will be begging the question at some point.