Those are hypothetical causes but they don't actually line up with all the observations. For instances, poor people are fatter than rich people. Shouldn't rich people be fatter since they 1) don't have to work as much manual labor 2) have more access to food.

You can try complicate the explanations to account for this but the fact is that it shows that the 'obvious' reasons most people surmise are not so clear cut and need to be tested like any hypothesis. It's a hard problem to test and there are several other competing hypothesis. The best I can say for my own beliefs is that we need to have more studies that need to be done before I'm convinced of any of them.