Health testing is relative new even in the show world of the American pit Bull terrier, by new im saying 10 years, with a boom of requiring health tests in the last 5 years.
The American Bully community is not as versed on the procedures and tests that should be used to determine breeding potential or stock. If the average American Bully owner has dogs that are under the age of 4 they may have not experienced a dog that is dysplastic, therefore the need to health test will not neccesarily be on their mind.
The majority of all breeders do not use health testing, in regards to hips and hearts. The statistics that say most fail, I would challenge based soley on the fact that few dogs have been tested to have an adequate sample size to pass a judgement. Health testing is also not going to elimninate dysplasia or heart issues, often times people breed sub par dogs based on the passing of health tests when the dog is structuarally not a dog that should be bred.
The American Bully does have breeders that health tests, as a member of the community I encourage health testing, although I encourage hearts over hips, if its one or the other.
To state that most would fail is based soley off of appearance and not science as I doubt very many have been tested. However, you show me the ones who have recieved a OFA poor, or who are dysplastic and I will show you some who have passed. I know of one very predominate Bully male who was dysplastic, but I also know of dogs that are 4 times bred on said dog who are OFA good.
The reason for the lack of health tested dog, is purely a lack of education and mentorship in the area of testing.