We know most people walk as their form of physical activity. We also know there are good relationships between lower body mass index and the use of public transit and walking, having destinations to walk to.
If you live in the suburbs, having nice trails to walk to will lead to an increase in physical activity, and if you live in a city, having access to all kinds of stores and things like that is what makes people walk. So when we construct suburbs with no destinations to walk to, people don't walk.