"Because they are big" Lol! Most people don't know what a true pit bull is even supposed to look like, they think pits are a 100+ pound monster with huge heads. Mine are 50 and 55 pounds with perfectly normal sized heads for the breed. I love how many people honestly believe they either have "locking" jaws or some magical amount of jaw pressure that other dogs don't.
Bad ownership and parenting cause the vast majority of "attacks" and seeing how most fatal attacks are on kids it really doesn't matter how big the dog is, heck that one baby got killed by a 6 week old puppy because they left them alone together and the puppy chewed like puppies do. But nobody ever seems to blame the parents.