Like most of the atheists here, I’m not “sure.” But many gods that people describe contradict observable reality and the ones that don’t offer no reason to believe in them rather than a natural universe.
Explanations of “god revealed themself to me, I have a personal relationship, etc” are indistinguishable from inner thoughts and/or mental illness and can be easily ignored.
I find this question basically the equivalent of “why are you so sure I don’t own a pet invisible unicorn?”
I’ve never been a strong believer (got taken to church as a child because it was the ‘social’ thing to do), but basically as soon as I thought about it I realised the God idea wasn’t acceptable to me.
I am quite prepared to believe there are gaps in our current knowledge, and I suppose it is possible that what fills those gaps may never be fully understandable.
However, I am fairly sure that those gaps won’t be filled by any entity, force, energy - whatever - that bears any relationship to any of the god concepts as expressed in any formal religion that I am familiar with.
That’s a fair summary.
Throughout history, supernatural forces and deities were claimed as the reason for all of the phenomena that we didn’t understand.
As we came to uncover the true answers they have never…never, turned out to be the result of god or gods. Such entities have never shown themselves as necessary to explain the world.
The concept of the “god of the gaps” is alive and well and the gaps into which they are being forced just gets smaller and smaller and more and more abstract and irrelevant.