In 1966-67, I became the Dallas Police Department's first active recruiter visiting colleges, small towns, and USOs near military installations. One thing I learned early on was that I could not recruit a college football player. The city's maximum weight limits for different heights were too restrictive. Any college football player, regardless of how fit he might be, was too heavy to meet the city's requirements. And I had no luck in getting those requirements changed because the old doctor who was director of the health department would say that they're in good shape now but when you put them in a squad car, they'll quit working out and get too fat.
I think most folks who read this thread will conclude, as I do, that there are really too many factors involved for there to be a simple answer as to why some (or so many) people are overweight.