You have licensed physical therapists, with degrees, who work through companies or hospitals and get paid very well, often with elderly people, and get paid by Medicare.
Then you have the personal trainer. That is a tougher road to go, it would seem. The physical therapist doesn't have to "sell" himself. He is needed by the patient to get back to walking, moving, etc. The personal trainer is training a somewhat healthy individual, and somehow must encourage results and keep up the cheerleading. That, in and of itself, is hard work.
And then there is the kid who just wants to get big & strong. He is very motivated, but has little money to pay a trainer for their time and expertise.