Why are Americans getting fat? Easy. Lazy, inactive, lethargic slugs will gain weight no matter what they eat.
Americans spend entirely too much time in front of the TV, at the "movies", at the "ball game", and in front of the computer. Even kids do it, in fact, these days kids are the worst offenders.
Americans do not play ball, they do not hike, they do not hunt, they do not fish, Hell, unless there's food or a game to be watched, they don't even go outside. They work desk jobs, or at work least inside, "protected" from weather and the elements. Too many machines and such have taken the place of physical labor and physical activity.
Exercise and hard work are your friends, not things to be avoided at all costs.