Q: Why are atheists so arrogant?
A: Some atheists are arrogant. Some are jerks. Some are rude, unrefined, condescending, and fatuous.
Just like every single other generalized group of people you'll ever meet, whether related to religion or garden clubs or PTAs.
Atheists are people just like you, some good and some bad. And if you frequent Internet sites frequently, odds are you'll run into more than your share of jerks of all stripes, atheists included.
But what generally motivates this question is when people run into a "strong atheist" who makes the positive assertion that "there is no God". This comes across as an unwarranted and unprecedented assault on the listener and all people who believe in anything spiritual or theological. I suspect it's this feeling of being assaulted that's behind the reflexive charge of arrogance more than anything.
People are entitled to their beliefs, whether it's in Xenu or animism or Jesus or the Aztec sun god. Or the belief that there are NO gods.
If it's arrogant for an atheist to proclaim that he or she believes there are no gods, why is that more arrogant than claiming that not only do you know whether or not there IS a god, but you know WHICH god out of the 2,000+ it is, and the exact manner in which he, she, or it wants to be worshipped? Why is one arrogant and the other is a moral exemplar?
Nothing in the statement "I do not believe any gods exist" is any more inherently arrogant than the statement "There is only one God, and Mohammed is his prophet", or any other equivalent statement you'll find in most religious or mystical traditions.
Because atheism is so rarely expressed aloud, I suspect its shock value is increased, well past the point of the other equivalent positive statements about the exclusivity of any given religion that we're all used to hearing every day.
The fact is that we're all probably wrong in some way about the universe, because it's infinite and we're finite. But we're all entitled to believe in something without being called names because of it.