1. Because kids are more sedentary these days. No regular PE until middle school, rides to and from school in cars, not allowed to go out and play.
2. Because fattening convience foods are so much easier in a busy life. It takes planning to produce good tasting healthier foods, and MCDonalds is easier.
3. Because our idea of portion sizes are so out of wack. Bigger is better is the idea.
4. Most adults view exercise as a chore, and say they don't have time for it because they are too busy and too tired.
My Dad has been an avid exerciser since he turned 40. In August he had some chest pains and was admitted overnight and then had subsequent tests. The determination was some sort of virus that caused inflamation around some of the heart vessels. What the tests showed was at 74, his heart is about the age of a 50 year old. His resting heartbeat when sleeping set the monitors off because they were set to 48 and his is below that. They had to really push him on the treadmill test to even get the heart up to a level to do the stress because he is in such good shape.
It is too bad people don't realize before it is too late the benefits of a healthy diet and exercise.