No we don’t!
Well, I don’t, and I don’t know any a-theists who do.
Mostly we are simply not convinced by the claims of theists. That’s not denial; it’s skepticism.
So when you hear a theist say that ‘Atheists deny god’, you are hearing a deliberate misrepresentation. A strawman. They would love us to deny god because that is an unsupportable claim (as is the claim that there is a god).
This is all tied up with the onus, or burden, of proof and whether it is possible to ‘prove’ a negative.
I contend that some negatives can be proven. “There are no cookies in this jar” is eminently provable – just upturn it. However, “There is no god” is a different kettle of fish. We can’t upend the universe and shake out any crumbs of a god. The universe is unimaginably vast so it is not possible to search all of it and come to the conclusion that there is no god anywhere. Whether or not it is possible to prove a negative depends on whether the domain in question is limited and fully searchable.
This is made worse for theists by their common claim that their god is immaterial/unnatural and, therefore, cannot be observed or, by extension, discovered. It’s almost as if they don’t want Her to be found! I wonder why…?
Then there is the problem that proof itself is not available in the universe. Proof requires absoluteness, which has never been found or made, so we have to make do with evidence. See my earlier article: https://www.patheos.com/blogs/secularworldbyaai/2020/07/is-there-such-a-thing-as-proof/
Nevertheless, as is the case with all claims like, for example, the claim that there is a monster in Loch Ness, the onus is on the claimant to provide supporting evidence. Popper said that science is a process of continual conjecture and criticism. If a proposition cannot be falsified, it is not scientific. Neither “There is a god” nor “There is no god” can be falsified. In the absence of such evidence, the rational, default, position is skepticism (disbelief).
So, why should we lean in that direction?
Because every mystery so far solved has turned out to have a material/natural explanation. Therefore, using Bayesian reasoning, an explanation involving a ‘supernatural’ agent is vanishingly improbable.
But that’s hardly denial, is it…