because the fear god
I agree with Dawkins that what most "believers" really believe in is belief itself. Most are clinging rather desperately to that belief and know at least at some level that what they believe doesn't make any sense. Most of them don't really believe at all but are terrified of non-belief. It shows in their actions. If bishops really believed what they claim wouldn't they be more afraid to face god for permitting child rape to go on for decades than they are afraid of facing the public or criminal justice? Would "Christians" be more interested in shopping than missionary work if they believed what they claim they do on Sunday? Yes, there are some true believers, but a lot more are clinging to their "beliefs" for fear of the alternative. Then along comes someone who claims not to believe. Non-believers are like the child in the old story who actually says out loud that the emperor has no clothes. It threatens them and naturally we all lash out and things we are afraid of. Personal attacks are only natural. My personal favorite is "well, you have to believe in something!" Really? Because I don't believe in things that haven't been proven to me. I might accept them as the best explanation based on our current knowledge and understanding of a topic, but I don't "believe" in the sense they are using that word.
Is your point that thinking atheism is arrogant is an insult led by fear?
It's a combination of ignorance and projection. They think it's arrogant because atheists don't believe in their god. Meanwhile they think nothing of the fact that they disbelieve in the thousands of other gods atheists also disbelieve in, while proclaiming their own god exists (with no evidence) and insisting his rules apply to all.