Philo, if you want to know why atheists act as they do, why not try to find out rather than just assuming you know?
You might read Dan Barker, who said that the very qualities that drove him to become a preacher drove him to become an evangelical atheist: he wants to explore what's really true and help others find out about it as well.
You might read Richard Dawkins' The God Delusion - on the very first page, he says:
"As a child, my wife hated her school and wished she could leave. Years later, when she was in her twenties, she disclosed this unhappy fact to her parents, and her mother was aghast: ‘But darling, why didn’t you come to us and tell us?’ Lalla’s reply is my text for today: ‘But I didn’t know I could.’ I didn’t know I could. I suspect – well, I am sure – that there are lots of people out there who have been brought up in some religion or other, are unhappy in it, don’t believe it, or are worried about the evils that are done in its name; people who feel vague yearnings to leave their parents’ religion and wish they could, but just don’t realize that leaving is an option."
You might read Greta Christina's book,
Why Are You Atheists So Angry? That'll certainly tell you plenty of reasons why atheists feel it is important to debate Christians.
And you skeptics should be honest about this. You're not just arguing for arguments sake, are you?
By all means, let me be honest. I am here for a number of reasons, and they do not contradict each other any more than it is a contradiction to go to a restaurant because you like the view, enjoy the food, and think it's important to help small family businesses.
I come here for fun. It's interesting to debate. It sharpens the thinking skills. It's amusing to point out logical errors.
I come here to learn. I have picked up a great deal at Christian Forums over the years. From Christians, I have learned interesting facts about the Bible, and about the details of different Christian beliefs. From atheists and other non-Christians, I have learned good arguments that expose logical flaws. From successes and failures in various debates, I have learned how to make arguments work, and developed my own views.
I come here to help others. There is no doubt at all that a great many people in recent years have deconverted from Christianity. This I see as a very positive thing, overall, and debates such as these can help in that process. Whether convincing a fundamentalist that they are incorrect to stick to a rigid and harmful interpretation of their religion, or convincing a liberal Christian that they are wrong to believe something if they have no real evidence for it, this is a good use of time, and helps the world in a small way. Converting people may not take place with the person you are in the process of debating, at least not straight away (although you never know). But more likely it is other people, browsing through the threads, who will be influenced.