I think it's because folks in other imperialist countries walk more, have a more natural/less processed diet, and watch less TV.
One big problem with American diets is that they include lots of super/processed foods with lots of calories and low nutrition. So you eat a lot and still feel hungry. There's also the long, long working hours and the long, long commute. A lot of sitting and stewing, getting home exhausted, dealing with the kids....and then bed. Exercise is largely the province of professionals and up (folks with flexible work schedules) and is the new "status" look. Like someone noticed that when workers worked outside; tan was out and white was in. Then when most workers moved inside to factories/clerical, tan was in and white was out, cause only the rich could afford to loll about in the sun. Now, skinny/fit is in (I have time to exercise) and fat is out.
This does not apply to Chuck Grimes who is in the best shape of any man I know despite the fact that he is in his late fifites and has a blue collar/indoor job:)
Joanna