When one thinks of a career, things that come to mind are job security, benefits, pension, health insurance, room for advancement, mobility.
A while back, in my early 20s. I did PTing in one of those big cities where people have money (NYC). I made decent money and met a lot of women, but there were problems. As another poster stated, the gym took a big cut of my money, and to make real cash, I had to make side deals with people, as did almost all the other trainers. That's not a good, secure way to make money.
My other option would've been to open up my own studio, but then I would've been dealing with big city rents.
I enjoyed being a trainer, but I wouldn't want to be doing it now at age 40. In fact, I often pass by gyms in the city and see the trainers. Most of them are in their twenties. It's actually sad to see an old trainer. You want to say, "Dude, what went wrong?"
I understand that you disagree, but then, that's what make horseraces.