Yes, I think those slimming centres are a waste of time and $$. Just go jogging and swimming everyday consistently for many months will lose weight already. And it doesn't cost much to jog and swim.

Back to the topic. I believe the trainers CAN make $$ themselves. They know what to do. They teach because they can earn MORE $$. Basically that's the main reason. Bill Clinton probably earns around less than $500k USD per year while in presidency. After he left office, he makes about $1 million per speach. And he makes many of them a year! That's the power of leverage. That's why he is a speaker. And that's why there are thousands of such trainers on the senimar circuit.