It's based on the lack of evidence and the courage not to clutch at metaphysical straws.
Right, thank you, atheism is based on the lack of evidence.
1) The atheist applies their human reason to the question.
2) By doing so they are assuming human reason is binding on the question.
3) Where is the proof that this is true?
Please consider what human reason actually is, and what the God claim actually is.
Human reason is the poorly developed ability of a single semi-suicidal species on a single planet in one of billions of galaxies. A species only recently living in caves, a species with thousands of hair trigger nukes aimed down it's own throat, a well known ever pending existential extinction threat it rarely finds interesting enough to discuss.
The God claim is a proposal about the ultimate nature of all reality.
So, this one little half insane species on one tiny planet in one of billions of galaxies thinks the rule system it has developed is binding on everything everywhere (a realm it can't even define in the most basic manner) and thus upon any gods who may or may not reside within.
Now it could possibly be true that human reason is binding everywhere. Just as the Bible could possibly be true.
But there is no compelling evidence to prove either.
Thus the entire debate is faith based on all sides.
And finally...
Theists generally get this.
Atheists generally don't.