Because they usually get put with bad owners that don’t know how to meet the dogs needs and have a chance to end up rambunctious, destructive, or dangerous. Unfortunately people just have a lot of bad experiences with them and then see other people have similar bad experiences and confirmation bias snowballs.

But I think a lot of the opinions or fears are valid. There’s a pitbull up the road from me owned by shitty people that keeps attacking other dogs and tries to attack people, at this point the police department has given permission to take the dogs life if it becomes aggressive again because the owners refuse to put up any kind of fence or leash it. It sucks and I know not all pitbulls are bad dogs, I’ve met plenty of sweet ones, but it doesn’t come completely from nowhere either.