The main causes as far as I can tell are a lack of giving a damn, a desire to eat more than the body needs (food's that tasty I sure can't blame 'em) and the fact that often the unhealthier foods taste a ton better then the alternatives.

But then again, some people also try to maintain a bit of fat for other reasons. It can give your body more weight, protects the organs and insulates against the cold. Granted they're likely the minority, but it is possible to be both fat and fit. All that takes is a regular exercise regime and enough food intake. And might actually be healthier then trying to remove all body fat in some cases. Too little can also be detrimental to one's health.