Some do, and there are certainly militant "atheists" who make a religion of their alleged non-belief, and attempt to make science a scripture. In my experience, precious few know much about science (and if i, who am not well educated in science, can spot that, they're in sad shape). These people attempt to beat others to death with their supposed excellent world view.
But this is a venue to which people come to argue such things. Most of the atheists here that i know of don't bring this sort of thing up, and most don't head for religious threads to argue. Even those who do are behaving in a manner which is not typical of their behavior in real life. Out in the world, i avoid the subject of religion. It's ill-bred, for one thing. People in society who wish to be polite do not bring up politics or religion as subjects for conversation. Another matter is that politics and religion are only likely to come up in conversation with fanatics. I have no interest in discussing things with fanatics.
It's an absurdity to say that atheists try to convert christians. I stopped practicing religion when i was 13. I stopped believing when i was about 17. But i was 40 before i ever encountered the militant, pushy, annoying kind of atheist and i avoid them like the plague.