*But I just don't understand why anyone would believe to know with absolute certainty that God doesn't exist
*
Your phrasing is a bit confusing but I know what you mean and it comes up on these threads time after time after time, so I'll address it.
There aren't many atheists around who'll say they know that no god exists. In my case, I am so certain of it that it feels like knowledge, but I freely admit that it isn't.
If anyone presents me with convincing, verifiable, repeatable evidence that any god exists I shall become a believer. It won't be a "choice"....I just will because I follow evidence wherever it leads me.
If you are asking why we are so convinced there isn't a god then the simple answer is because theists have not given us a single convincing reason to believe that there is.
The ball is in the theists court here, not ours. You bear the burden of proof as the claim that a god exists is yours, not ours. Our atheism exists only as a counter to your theism...without theism, there would be no atheism.
We do not live in a world where a god is self-evident & so belief should be the default. It's quite the opposite, in fact. And so anyone seeking to claim that there is some supernatural prime mover needs to demonstrate a sensible reason to give the idea credence. But no one ever has.
All we are asking for is evidence.
But, to demonstrate a bit more, imagine I told you that I had a tiger living in my garage. Would you automatically believe me? No...you'd ask me to show you. What if I refused to show you, or provide videos/photographs and told you to just believe me? You wouldn't. I doubt you'd tell me that it was impossible that I had a tiger in my garage, but you wouldn't actively believe it without evidence.
And, given that it would be most unusual for anyone to have a tiger living in their garage you'd be highly sceptical to say the least.
It's that simple.
*imo? Because humans have the ability to make choices
*
I didn't choose to be an atheist. And you saying this suggests that you think theism should be the default position. Why do you think that?