Food in the US is cheap compared to how much they earn. They also eat fast food often, which is cooked on the oil and contains a lot of fat.
It's not the fat that's making you fat, however that sounds. It's mostly carbohydrates that give you energy which you are unable to burn. The math is very simple: you consume 2000 calories a day and burn 2200 = you're getting lighter and thinner. Also, you need fats to be healthy, it's just that fried food tends to have more calories in the same size meal, so people lose control of how much they're eating.
Well americans love fast food basically. Everything about these unhealthy foods can make anyone fat. Its also about the lifestyle. I don't know why but many americans seem to be having a hard time burning all the calories and carbs they took. Probably because of a busy schedule but health should also be taken into importance