If you understood what the word theory means in the context of science, I wouldn't need to, but how about this:
There is no evidence of any being like a deity, hence the prudent course of action is to assume no such being exists until proven otherwise.
It's either that or you have to lend the same amount of credibility to the existence of unicorns and leprechauns as you do claims of gods' existence.

And for the record, we don't even have to come up with arguments.
The burden of proof lies on the one making the assertion.

And to get an idea of why people call christians closed minded, and why the atheist position isn't closed minded, how about you watch this instructional video?

www.youtube.com/watch?v=T69TOuqaqXI

You're the one who's calling people closed minded here. And you're doing it with zero justification, even.
Being atheist is about assesing the evidence available to you, and setting up a logical stance in accordance with this evidence.
Being theist is about looking past the lack of concrete evidence, and taking up a stance based on wishful thinking and faith. In taking up such a path, you decide that this path is the correct one, and you are no longer open to new ideas. This is truly closed minded.
An atheist, on the other hand, is open to reasses their position in accordance with new evidence that may arise. If some proof beyond reasonable doubt of gods' existence arises, an atheist will accept this evidence, and adjust his stance accordingly.
That is called open-mindedness

Maybe they feel the need to defend themselves because they are being attacked?
Believe it or not, atheists are something of an oppressed minority in some areas. Sure, it's nothing like segregation, but they are still rated as less trustworthy than rapists by the American public. And good luck getting into politics in the US if you confess to being atheist.

Or maybe the reason you feel atheists are so vocal is that you're on a bloody public forum. You know, an area designed for discussing viewpoints, or making jokes about things.
How is it not natural that one would come to make jokes about, or discuss (or ridicule) religion as well?