Many (most?) people have a fundamental need to feel superior to some "other". This need is strong enough to the point where they'd rather be doing poorly themselves so long as there is at least some other group that's doing worse. Were you to give them an opportunity to significantly improve their own lives but in a way that everyone ends up on the same level, they'd balk.

Obesity is one of the few conditions that are socially acceptable to openly mock, and as such, it's like catnip to people who need to feed their inferiority complex.

Cruel indeed.

(Don’t want to try to find a source for this - consider it as hearsay)