The big bang doesn't answer that afaik since it requires a singularity (I think), which is something (I think).
> Well, why not?
-- Stanford Encyclopedia of Philosophy, "Nothingness" (https://plato.stanford.edu/entries/nothingness/)
Either there's 'eternal' nothingness - an option which we have excluded empirically - or something always must have existed. We're trying to work out the nature of that something, and the meaning of 'always' (did time begin at the big bang, making the question of 'before' as meaningless as the question of what's north of the north pole; is our universe just the lastest stage in a possibly eternal cycle; does the singularity hide a mirror universe with an arrow of time pointing in the opposite direction; ...).
Physics answer "How" things are, if you want to know "Why" the answer is to be found in philosophy or religion.
This particular question also seems most nonsensical to me - nothing can be said about a "universe" where nothing exists, so why even discuss it?
Why are there stars in the sky?
Nothing could be said about stars in the sky if there were no stars in the sky, so why even discuss it?
etc ad infinitum
But the phrase "something exists" is tautologically true, while the phrase "nothing exists" is a logical contradiction (since the phrase itself and the speaker and the listener must exist). So it's logically impossible for the whole universe not to exist, as long as we can have this conversation or even thought. It's like discussing why 1+1=2 and not 0.