There are estimates that we daily walked for 10 - 20 kilometers for hundreds of thousands of years. The world's best problem solving machinery grew up under conditions of consistent, strenuous physical activity. It makes sense that when we don't recreate the environments in which the organ was forged, we get a loss of function. And that when we do restore those environments, we get that function back. The effects of aerobic exercise on executive function skills is a powerful empirical example of this idea.