They were bred explicitly for fighting. First fighting bulls in pits, hence Pitbull. That was outlawed. It was deemed unfair to pit different animals against each other in a fight. So pitbulls were then bred to fight other dogs.
Pitbulls were killed when they wouldn’t fight, or were beat by another dog. The breeders didn’t care about them bitting humans. They wouldn’t keep them as pets as they were for fighting.