I used to wonder about this myself. The reason why fitness is so prevalent, is because the government, and corporations too, push health and fitness ideals and information. So, people know a lot about it. However, knowing and application vary significantly. People, lack motivation or discipline to get into shape and just give up or only give a half hearted effort.
As for obesity, I think it has to do with inactivity and diet. People in many areas have a very low activity level and this contributes to overweight individuals. More than that, the American diet is rich in fat, and toxins, found in processed foods, which encourage fat production and retention.