lifestyle changes like you envision are notoriously difficult. if I wanted to single one out, "eat less" is clearly non trivial, since every fat person ever is aware of it and has tried it, but "trying" is not the part that gives the result.
Overeating is an artifact of some other condition(s). Very few people stay fat or thin their entire lives. People fluctuate over the years based on where their head is at. I've done this myself.
All that said, the changes to diet and exercise really are trivial. It's just that people choose to express their mental state this way (possibly from ignorance). It's an arbitrary choice.