Ex physicist here: Fucking no clue, but here's two neat ideas
Because there has always been things. Basically it's entirely possible the universe just kind loops around given enough time, there are a few really interesting ways to do this but the classic one is where the big bang reverses and there's a bug crunch before a new big bang. That's not very likely based on our observations, but there are other more mathematically complex ways to have a cyclical universe, and they don't necessarily require having a defined beginning.
Because nothingness is unstable. Basically, if there's a concept of nothingness, no energy, particles time or space, but it's possible for little universes to occasionally exist and disappear really quickly, then it's possible that our universe suddenly popped into existence, got really fucking big before it could disappear again and then got stuck existing. This is based on the highly advanced area of physics called making a wild fucking guess.
I'd say most likely that we'll have to be satisfied with that not being a question that can be answered. Much in the same way that we can't answer the question of why the laws of physics look the way they do, we can just describe what they currently are.
There's a third option: Black holes create new universes through some as yet undiscovered process. Then your existence just becomes a statistical eventuality, as do every other life that you could ever live.
There's a fourth option: every reference to the mystical properties of black holes on lemmy creates new universes through some as yet undiscovered process. Then your existence just becomes a statistical eventuality, as do every other life that you could ever live.
Yeah that's what I was getting at, all we can do is guess. It's pretty easy to realise it's impossible to answer scientifically, anything that could have any impact on our universe must necessarily be part of it and so cannot tell us anything about what came before.