But what you eat and what you do are massively influenced by your environment. Rare person who can resist the bombardment of ads from the junk food industry.

It's due to inactivity, look it up. Or as WHO puts it: "Overweight and obesity result from an imbalance of energy intake (diet) and energy expenditure (physical activity)."

I have seen too many overweight people do too much useful work to believe that.

It is not like your time in gym or being obsessed over diet would be useful to anyone else. They are you having hobbies, to large extend. An overweight person spending that time actively engaged with kids or on the job work more, by definition.

Overweight - sure. Obesity - mostly due to psychological issues, not laziness.