Cafes and restaurants are another. You wouldn't believe how many clueless yuppies start up thinking they'll spend their lives sitting around drinking wine with customers who are all their bestest friends.
Clothing and "craft" and homeware retail's another. Check out Toorak in Melbourne or Bondi Junction in Sydney, heaps of rich blokes' wives with little shops.
Happens in every industry. The main difference with fitness, I suppose, is that we have 18-25 year olds doing it, usually it's older people. They're all convinced they'll have six figure incomes in the first year. "$60-$100 an hour times 40 hours times 52 weeks..."
If you're really really good, start your own business, you'll make more money and have more freedom that way. If you're just good or okay or crap, work for someone else. Of course, everyone thinks they're really really good,
especially the ones who aren't. Oh well.