It's not. We invoke the word 'God' because it has general usage...

It's not. We invoke the word 'God' because it has general usage and kind of has a universal meaning, but by all means choose some other word.

We have no idea what God is, at best it's a name we give to some force, energy, external to physical reality that directs, coordinates, limits, adds purpose, etc. Humans play ball with it by universally believing it exists in some form, is the Cause, or first Cause, or the ongoing Cause. Every tribe, culture, nation has built their existence with some form of this Cause as the focal centre of their culture, that not a small thing to just dismiss.

And what's the alternative, a cyclic system, okay I tend to think it is a cyclic system along the lines of Hindu Cosmology, but even then some cohesive, directing purpose surely still needs to be acting on the system or we are accepting that matter / a universe and all therein, is self-causing, self-organising and the producer of life and cognition. I don't see how that can be the answer either.

No system causes itself before it exists then applies exacting limits and has life and cognition as a byproduct. There is absolutely no need for anything to evolve, let alone exist, the ability to evolve need not be an aspects of the very first organism and yet it was and for that single celled thing to become human and everything else just because of vast amounts of time, competition and changing circumstance is a bit too much like literal Genesis for me.

How can there be a natural explanation to a universe existing? We know enough to know that the universe can't possibly have caused itself. We know this because every single thing in this universe has a cause so the universe is a no more than a vast collection of effects.

Give it a go by all means but it's beyond me, I can't fathom any way for a universe and all the junk in it to exist without some kind of external agent, as annoying and unsatisfactory as that might be.