A LOT of it has to do with how they are raised. And many breeders don't care. Many owners are idiots.
My brother had one. We had a few people in our family who have dog training experience. So he talked to them. From the time it was a puppy, he trained it.
He made sure it didn't get upset when food was taken away. He could take its bowl away and it would just sit there and look at him waiting patiently. Never had an issue. He introduced it to other dogs so it played with them fine.
His only regret was not introducing it to children. A friend took the dog and it stayed at her daycare. She wasn't sure how it would do with the kids and he was nervous too. It had never shown aggression or anything but they decided to see how it went. He called to check in later that day. When the kids arrived, the dog wasn't sure what to do with such little people so it got scared, hid under the kitchen table, and peed itself.
Now of course, they introduced it to the kids and he figured out they aren't a threat. And the kids just climb all over him.
There was only one time they thought they had an issue. The dog and current owner were in the backyard. A neighbor had a large, rather disagreeable doberman. The doberman apparently decided it was a good idea to jump the fence and attack the pitbull.
It happened so fast neither owner could stop it. The doberman jumped the fence, growling and charged the pitbull. The pitbull grabbed the doberman by the throat as it attacked, flung it up in the air and then back to the ground by the throat.
The owner of the doberman came running screaming in terror thinking his dog was dead and the owner of the pitbull came running as well. The pitbull released the doberman, jumped up, and bounced around the doberman trying to play.
The pit actually thought they were playing. It didn't have any concept that the other dog was actually trying to attack it. It had no intention of hurting the other dog and there was barely a mark on the doberman's neck.
The dog was just raised in a such a way that the need to defend itself or attack someone just doesn't seem to enter its brain. Its worst trait is defending the household loudly. It growls at anyone new until the owner says its fine. Then its back to begging anyone around to play fetch or rub its stomach including the poor new person who gets a new Pitbull friend whether they like it or not.