I don't know why Americans are so fat. But I do know why I'm overweight. Its a combination of my genes and lifestyle. The people on both sides of my family are big people, and I don't necessarily mean fat. I mean we usually come out mesomorphs or endomorphs. And not to knock my black kin but the black community has terrible eating habits...we usually don't care though, we typically favor a nice big body.
My eating habits as a child was atrocious. No one bothered to tell me what my problem was unless 5 years of "Your a fatass", "haha, Cartman", "damn, you need to lose weight man!" was just people's way of saying cut the fast food, eat less, and exercise.
I eventually grew into my big body but I still have poor habits such as hardly eating. When I do, its usually fast food, or just something non-nutritious and essentially meaningless. I like walking but any other kind of exercising exercises my patience.
As for most Americans, I believe it has much to do with our fast paced society, fast-food franchises on every corner, negligence of watching what we're eating, television, cars, shitty processed genetically modified foods, etc.