If only doctors could prescribe it and have people adhere to the prescription.
If only American culture weren't so car-centric and our daily lives involved some natural amount of exercising. We've built a society where only the privileged few can walk or bike to work. It's just too easy these days to barely move in our normal life. Drive to work, drive to the store, drive the kids places... you literally have to specifically schedule exercise (and ironically, many people will drive to the gym to walk on a treadmill).