Praise and blame are not just alternate possible ways to treat a disease.

Eating and survival are fundamental functions of life. Someone whose regulatory systems are so out of whack that they are eating/fasting themselves into an early grave, is probably subject to control dysfunctions which have inbuilt advantage over intellectual or social control.

Also, punishment is the trickiest of all behavioral modification techniques. It is very likely to backfire, which makes perfect sense. If punishment was very effective on a given individual, he/she would be a perfect slave. Being a perfect slave isn't so great from the perspective of the slave, though it is good for the master. Since human biology doesn't make it easy for a large population of slaves to be related to a master, it makes perfect sense that we'd evolve defenses against punishment.

For what it's worth, a member of my band is morbidly obese. He has taken extraordinary measures in terms of effort to lose weight. (Eschewing use of a car in Houston and walking everywhere instead.) His condition is not voluntary.