It's not, it is against fundamental animal/human instinct and hormonal response developed during evolution: eat as much as you can when you have access to food.
Don't you think it's a bit less simplistic maybe ?
probably more Americans spent time physically working, and not sitting 10h/d in office and another 2h in the car commuting.
> Especially rich folks who have access to infinite amount of food
I am sure there are obese rich, also rich likely in general have more willpower to control their instincts, and that's partially how they become rich.