I think it's portion sizes. Portions here in the US are HUGE compared to other countries. I think I read somewhere that when eating at McDonald's in France, their large size fry is the same size at our small fry here......that's a huge difference.
And also, everyone is always on the go, no time to stop and cook, savor our food, nothing. Fast food and big portions is what it's all about here. It's pretty gross.