Why are Americans out of shape?

I am on a fitness kick lately, which started me web surfing about statistics, which led me to discover how shockingly out of shape Americans are. As I was web surfing I came upon several theories in short order. Some blame food choices available, others indicate our work week is so much longer than other industrialized nations. I also found that those of Asian descent are considered to be naturally smaller and have lower BMI ratings, making me question BMI for Americans vs. Europeans.

What is your theory about Americans and their health choices?