Because the alternative is impossible.
Of course, so too, is any concept of god itself.
Ironically, having no existence whatsoever, forever nothing past and forever nothing future, is the ONLY possibility that makes sense. Yet, here we are. The impossible, whichever way you look at it.
I don't understand. Why would either God's non-existence, or His existence, be senseless to you? I can make sense of both.
From one (pretty logical and straight forward) perspective, God or the gods sorta punts on the real question.
God is the "answer" - why are we here? how did we get here? what happens before we live, after we die? What is some common thread to a universe that has as more stars (or is it galaxies?) than there are grains of sand on the earth's beaches? What is "consciousness"
God provides a convenient answer to all. But it's not , really, an answer. It's the equivalent of asking how did we get here - without any answer that actually works - so you say, I am here because God (or the gods) put me here. Ok... so where did god(s) come from? We are just putting off the question, as if this "God" becomes a catch all.
To me, I look again at two impossibilities. One, that we were here forever, or that we suddenly "formed" as a universe from nothing - again, neither of those options seem possible. Two, that there was something (God) that "created" everything or that things just, well, happened. But remember, if it's the former, you are still really stuck with the latter (God existed forever, or "just happened)