They do, atheism is a lack a belief in gods, not the belief that gods can never ever exist.
Atheism is just the natural position you are born with, you have no belief that god exists until you are aware of what god is and what proof exists.
It's totally safe to say that atheism by definition is nothing more than not believing in gods, it is not necessary to totally deny that a god could exist, but there is absolutely no reason to believe a god exists either.
And then there fucking assholes like agnostics come and say "no, we think god can exist but we don't personally believe it exists", well guess what you fucking agnostics, you just defined atheism.
Then there's religious people saying atheism is a religion, fucking hell, I just give up at this point. You can all call yourselves all you want, I'm tired of this shit. Atheism = lack of belief in gods, for various reasons, it's not my personal problem if someone says god can never ever exist under no circumstance, that's their opinion