Perhaps the main reason that there aren't many atheist hymns is that we aren't an organised belief system, and most of us never talk to other atheists as such, just as people. Atheism is a private matter. And we think those with other belief systems should have the common decency to keep quiet about theirs too, and not organise to pass laws and engineer social situations that impose their values on others. BBC's Thought For The Day is one example, the laws about religious education and the promotion of faith schools another. They might just be starting to regret that last one.
A friend died a few years ago having asked her children to give her a humanist funeral. It was embarrassing, the children had left it to the undertakers to organise it. They were clueless. A woman dressed in a short skirt and tights (black), a sort of red hunting jacket, and a top hat, assured us all that our friend was now happy and looking down on us, forgivingly. She then recited the (apparently non- religious) Lord's Prayer, complete with the kingdom, power and glory. The irony was that our atheist friend had been a Catholic when previously religious, and they don't do that bit.