And how is that different than someone saying, "I think there is a God."
Because the belief in a God comes with a lot of baggage. It automagically implies religion, and the possibility that one can commune with the supernatural to gain insight or knowledge that one otherwise couldn't know, and perhaps also the possibility to get that God or those Gods to intercede in the real world on the believer's behalf. Humans are innately logical creature in some ways. And so, when someone believes in a God, and they pray for their God to intercede on their behalf, perhaps by making it rain so all the livestock doesn't die and all the crops don't wither and die, and then it does rain, well the human will naturally put those two together and assume that it was the prayer that led to the rain. Next time drought threatens, the believer will probably try to replicate the prayer that worked last time, therein creating ritual. In a lifetime, you can go from only the vaguest belief in the supernatural, to a full blown religion.
This sort of thing doesn't happen because of the disbelief in Gods. What builds from atheism? A critical view of the world, a skepticism of those that claim religion and/or that the supernatural is real. And frankly, many people don't even go down that path at all. In today's America though, you do tend to find a little more commonality among atheists, because we're derided by the majority, and have to defend ourselves regularly from them, if we make the mistake of letting it be known that we don't believe in God. There are huge swaths of the population that automagically assume that atheists are deceived fools, liars, or agents of Satan. So there's positive reinforcement for atheists to learn something about science, biology, genetics, geology, and so forth. There's pressure to learn something about history and philosophy as well. There's some pressure to learn rhetorical skills as well. There's also some pressure to learn self defense as well, because sometimes pissed off crusading Christians will physically assault an atheist. But all of this is because some segments of the Christian population won't shut the fuck up and leave atheists alone. If it weren't for that, most atheists would just leave well enough alone, and wouldn't feel the need to go poking at the religious. But because atheists get treated like shit by the religious majority, it creates a lot of resentment in some people, and they push back, say by putting up signs next to a nativity scene that say that religion is bunk.
There are also huge swaths of the population that make the mistake of believing that atheism is equivalent to religion, and so their knee jerk reaction is to believe that atheists get together in clubs once a week to worship Charles Darwin, to memorize holy text from Darwin's writings, and to sing Korn songs. They imagine that there's this community of atheists that can lean on each other, support each other, and do all those other things that religious people do at church. But, in fact, atheists don't have anything like that. We don't share a positive belief that draws us together like that. All we share is a negative belief, one that says, "I don't believe God exists." What would an atheist club do
? Sit around and bitch about religion? That sounds... really fucking stupid. Doesn't sound like a good time at all
. Other than that, I suppose they could teach and study genetics, geology, biology, upper mathematics, statistics, philosophy, and so forth. But I've already been to a place like that. It was called college. It was a lot of work. Most people won't go back to college, even if it's free, if they're not going to get anything other than knowledge from it. So... atheist club... not really a go.
On the other hand, it might be a lot of fun to get a bunch of people together to sing Korn songs every Sunday.