Because they are purchased by people who are uneducated and uninvested in their training/socialising. They just want to take photos of them and feed them mass gainer food and walk around the local estate with them on a thick chain whilst actively teaching them to appear aggressive if need be, but with no idea about impulse control.
Or that is who owns them around here.. I have never had a problem with an American Bully, but then I wouldn't have someone in my social circle that doesn't bring up their animals correctly.
As a side note, the brother of a good friend of mine has a 1 year old entire working lines retriever who they bought a week before their baby was born - they got seriously lucky that he is amenable and never went through the nippy phase really. It's a shame he is completely unruly and untrained though as he could be a fantastic dog, so clever and quick off the mark with things. Well idiot girlfriend of the brother, who wanted the first puppy, got jealous that the dog likes the boyfriend more so bought a German Shepherd puppy to be 'her' dog.. so they have a tiny house, they don't walk these dogs, they are completely untrained, and they are both entire. Unfortunately, even at 10 months old I can see that the GS doesn't have the bite inhibition he needs to be safe around a 1yr old child and there is the early 'play' test for dominance but my warnings are falling on deaf ears... I hope this doesn't become another news case one day.