Because so many of them spend an inordinate amount of time on a personal crusade (pun intentional) to attack something they claim doesn’t exist - an off-putting combination of irrationality and antisocial behavior.
Most doubt the existence of the Tooth Fairy but they don’t spill oceans of real and virtual ink writing to convince others of same.
Which means there is another agenda at work.
Of course that's another argument that shoots down the idea that atheism isn't a religion. There'd be no such needs to "evangelize" if that were not the case.
My son says "He believes what he believes, I believe what I believe." They don't discuss those beliefs. They spend their time together talking about the things they do have in common. If either spent time trying to "convert" the other, their friendship would have lasted about 5 minutes.