I have never been a PT but my observations at various gyms that I've belonged to have led me to think that many PTs prey on the ignorance of inexperienced clients and feed their fears about working out "incorrectly" in order to get the client to keep paying them week after week. I go to a Gold's in western Massachusetts and have never once witnessed a PT showing a client how to do a freeweight squat, bench, or deadlift. Seen plenty of them showing their clients how to do one-armed curls while balancing on a BOSU or needlessly micromanaging every aspect of their workout to make it as complicated as possible, and thus as difficult as possible for the client to do it on their own without the PT. Like yesterday the twink head PT at my gym was having a client do pushups, nothing wrong with that, yet barking instructions the entire time. "Tilt your head up, down a little bit more, there it is. Move your hands in and back a little, not that far, up a little more..." Fuck man just let the dude do a pushup, it's not rocket science.
In closing I'm sure there are good PTs out there who genuinely want to help people lead healthier, fitter lives, but I think a great many of them are in it because it's a cake job that requires little actual training or intelligence, and couldn't care less about actually helping someone improve their fitness.