Usually, they just don't see evidence for a God, or greater force, and so they do not believe, likening the belief in God to a belief in invisible green hobgoblins or such. Understandable really, since those drawn to the the need for evidence (skeptics) will find no hard evidence of God. Although a true skeptic will understand that there is a slight chance God exists, for the sake of simplifying one's beliefs, and not getting caught up in 'what ifs', does one just go with what they feel is the most probable theory, and do away with the others (a big objection I have with the black and white thinking fundamentalists).
Atheists get slammed for being immoral, but I think that is just an old stereotype that should have been laid to rest decades ago. Atheists were often the red-light district patrons, or people that threw off what was considered moral by the Christian majority. But since these days Christianity is hard put to to justify some of it's beliefs, have atheists been on the increase.
Growing up in the shadow of a vengeful Yahweh, one that will cause eternal unceasing torment for a wrong done (up to and including a false belief, as some Christians will denounce other denominations to be of the devil), have many found relief in atheism.