Because...
..."nothing" doesn't exist. Empty space is not nothing it is empty space. Primordial matter is not nothing either. After a theoretical "big crunch" you still have something and before a "big bang" you must ALSO have something. At no point does nothing actually enter the equation. Even the concept of nothing is something...making even the concept of nothing an oxymoron.
It's a common conceptual fallacy that "nothing" actually exists.
The fact that you are here...means existence is here...and nothing can't exist.
If none of us were here and nothing else as well, then nothing would still not exist...
...either way we come up with something.
;-)