Why would america have such a high proportion of obese people?

Food and Cars: Compared to 50 years ago, we now eat much more food at meals, we snack more, we consume more sugar and carbs, and we eat out more at fast food places (where the portion size has trebled). Food is cheaper as proportion of income too. We also drive more, take elevators and escalators and not stairs, spend more time sitting watching tv/computers, and have jobs that are more sedentary.