I would imagine that most nonbelievers would term themselves as 'agnostic' more than 'atheist.'
It's not that they deny the possibility and existence of senescence beings beyond us, they just think that in light of what we know, the previously agreed to stories, folktales, superstitions, myths and texts don't seem to ring true. It would seem that the vastness and complexity of the universe a god made in lovingly in our image and likeness ain't cutting it anymore. At the very least, it's highly unlikely that what ever created both DNA and the universe we know see through Hubble and Chandra is spending their spare time writing - or dictating - books to humans.
And given the history of martyrs, pogroms, inquisitions, crusades, jihads, witch trials, suicide cults and suicide bombers, not to mention people who follow a prince of peace, or a religion of peace that have their own armies, people who claim that god hates drugs & gays and have meth habits and gay hookers, people who swear to poverty and live a live so lavish that kings would be shamed, there is not exactly a shortage of things not to like about the entire concept.
And, at that, I bet he 40 million number is way low. Lots of people are afraid for saying so, and lots more 'believe' as some sort of business networking device than anything based on actual faith.