recommended reading about the environmental factor hypothesis

https://www.lesswrong.com/posts/NRrbJJWnaSorrqvtZ/on-not-get...

it doesn't have to be just habit, but cultural/learned factors likely play a huge part.

each generation eats more, gets bigger, obesity runs in the family, (many obesity associated genes are mostly expressed in the brain ... the typical cursed environment-gene interaction), portion sizes got bigger, it became normal to gulp down a lot more sugar, etc.