Because Westerners tend to eat more calories than they burn off habitually over time. Typically because they can't deal with the emotions and feelings they experience of being hungry while dieting to lose weight or not being able to use food as a means to avoid challenging emotions. They also tend to be very insecure about their weight issues and will angrily lash out at perceived criticisms, real or imagined.
If they knew they'd eventually get used to eating less and habituate/transmute their challenging emotions by allowing themselves to feel them, they might have more luck maintaining a reasonable weight while eating a reasonable diet.