When the former CFO of Weight Watchers is asked in an interview how a company can keep existing if only 16% of their clients are succesfull (because remember: weight loss diets don't work in the long term for the large majority of people, not even Weight Watchers!), he answers:
"[It] is successful because the other 84% have to come back and do it again. That's where your business comes from."
And there you have the business model of all diet companies.