Cuz they feel like they're banging their heads against a wall.
I don't really fall into the category of atheist or believer, but I'll go where the facts lead me. That generally leads to atheism, and is why atheists end up getting mad and therefore rude at believers: They will lay out facts, raise questions, play devil's advocate, and reason in an evidence-based, structured way. I've seen it many times before on this forum. Well folks, I've been trying to reason with a JW.
It goes round in circles. Anything too hard to answer is ignored or "I'll have to do research on that." They pipe in about how they must be Truth because of the Big Three doctrines, hellfire, immortality and Trinity. You point out facts from independently verifiable sources. Then they accuse you of mockery and negativity. Then say you are unappreciative of what Jehovah did for you. Then it starts again. Something from the Watchtower. Something in circles.
Makes me wanna tell him to STFU and say, "fine, you stupid #%&@, have your [email protected]%$&*#!!! cult!!!!" Not exactly persuasive or logical argument, but you get so mad!!!!
Now I understand so many of the less-than-logical things posted here by atheists.