[logicchains]

We know exactly why; the average American dietary caloric intake has increased to three thousand and something, which is more than the average person burns, especially if the person is as sedentary as the average American. More calories eaten + less exercise = more weight gained.

[phkahler]

This is the technically correct answer and we should try to address it from both ends - eating fewer calories AND being more active. There are a number of issues with "just eat less" because there are reasons people eat more than they should. Part of that is what they eat (what you eat matters for multiple reasons) and also habits or other psychological reasons.