Fighting dogs were bred with animal aggression NOT human aggression; it would be unsafe for the handlers to break up a "match" if the dogs were human aggressive.
Dogs with poor training can be retrained; dogs with poor genetics have to be managed.
These two items explain why so many seized fighting dogs can be "rehabilitated" to be pets; as long as the new owners understand the encoded animal aggression.
Many of the problem pit bulls have been described as being loving pets that without warning became human aggressive; this is not the description of a temperamentally well balanced dog from a breed/type developed for animal aggression (with no human aggression). Temperamentally well balanced dogs don't go from loving pet to killer without warning.