Well when people are not making false assumptions about things, it is easier to carry on a conversation.
Okay, which assumptions specifically are false? You keep saying "You're wrong!" but not explaining how... that's not productive.
Since when was it necessary for someone who freely chooses to believe in God or follow a certain religion explain why?
Well on one hand this is a debate forum... usually people in a debate must clarify their side and the foundations for that side.
Secondly, atheists doubt theism, so the foundations of theism are especially in the spotlight.
Third, to understand why people are atheists you have to understand that they don't feel theism is adequately justified. Thus, atheists request justification for theism to see if there's something they've missed and to try to further their understanding.
Otherwise you get something asinine like this:
Leprechaunist: What's the best argument for not believing in leprechauns?
Skeptic: Uh... well, the lack of justification for the existence of leprechauns. It's sort of like believing in Santa Claus as far as I can see.
Leprechaunist: That's ridiculous and stupid! Comparing my belief to Santa Claus! You're an idiot!
Skeptic: Well, can you explain to me how the two beliefs are different?
Leprechaunist: Don't be absurd, I don't have to justify my belief to you!
(repeat ad nauseum)