The problem is that the population which is not doing physical activity is the population uniquely at risk for overtraining. You can't put a couch potato out and have them run, run, run without them ending up with shin splints, stress fractures, and all the other issues that come from pushing too hard, too fast.
Saying we need to do more physical activity is not carte blanche to run yourself into the ground. No decent running coach would recommend running every day anyway. 3-4 times a week, and then let your body rest and recover. Training does not make you healthy or stronger. Recovering from the training makes you healthy and stronger.