They were bred specifically for bull-baiting and dog-fighting for human entertainment–both those blood sports require aggression. They’re not a very old breed. Their breeders used to kill (cull) the ones that showed aggression towards people, and selectively breed the ones that were good pit fighting dogs. That’s how they earned the reputation of being a good family dog for a time (provided a family didn’t have a cat or come into contact with other dogs which the pit bull hadn’t grown up with); the dogs were usually reliable around their family if they’d been well trained and socialized.
Now, though, they are bred unscrupulously by some people, and human-aggressive genes are part of their bloodlines. I’m sure some bloodlines have been more carefully bred, but for the typical, sympathetic would-be dog adopter or buyer, they wouldn’t know the full genetic background and socialization experience of a puppy or adult pit bull they may take home.
And that’s where much of the problem arises, IMO. This may sound cynical, but** it’s the people I distrust, not really the dogs per se**. If I’m out walking my Shetland Sheepdog mixed dog, and I see someone coming along with a breed that has a history if being bred for dog fighting, to be blunt, that person is unfairly asking me to have a huge, unearned amount of faith in their dog-handling skills. ( With pit bulls, in this instance, even their most ardent defenders admit that the average pit bull is not as safe as the great majority of other breeds around dogs and cats.) The owner is the weakest link in the chain that’s keeping my dog and myself safe around a dog like pit bull. I’ve unfortunately learned from experience that kind of faith is misplaced, and I will never count on someone–especially a stranger walking their dog-- to be able to control their dog when the dog’s breed was created for fighting.