1. Why would anyone expect there to be an answer to this sort of framing of a “why” question?
2. Given that the current observable situation for humans is “here we are, there those things are”, why do we keep framing this question as if it’s surprising that things exist?
3. Why would the starting situation for a universe ever be assumed to be “in the beginning, there was nothing”? (Except that, of course, many human religions say so… interesting that they all do…) But what if there “was always” something in whatever the universe and time are? It seems to me there needs to pre-exist some context in which a universe can be created into, if one needs to frame the idea that way.
4. God creating a universe is a metaphor, not a literal thing, in every religion, when understood at the level of its philosophical intent for people who understand it. “There was an actual God, and he made stuff” is just something to say to get unsophisticated people to stop asking questions. God is a metaphor for the universe in its totality. God made the universe and has a plan for every atom… because “God” includes the entire universe and everything about it… God is not a dude with a beard somewhere, or even that sort of thing, at all.
5. If you ask how God came to exist, you’re asking how the universe came to exist, but universe is a universal concept, so it’s a pretty self-referential question from the literal point of view the question seems to be asking from.
6. A related (or possibly the same question), is why your consciousness exists, and why its attention is on the person and time it is on.
7. Many religions at their mystic/guru level lead to a relationship to the universe as an illusion, and the higher level of reality being a god consciousness which is the consciousness we all experience and is the only thing that actually exists.
8. Some philosophy and physics and mysticism suggest that there may be a multiverse or even a continuous infinity of universes with different natures and timelines. What that even is, and what might exist conceptually above that at even more levels, it seems to me, is in a context outside what we think of as “the universe”, and we don’t know what that is, but it seems like it might be necessary to answer “why” or “how” the universe came to exist.