This is the key. We like to blame people for being inactive and overweight, but we've designed our transportation and food systems so that only the most motivated can be anything else.
Whenever I visit family in the midwest it can be a bit of a culture shock: by default you might find yourself walking a few hundred feet per day. With garages built into every house, few sidewalks and office parks/shopping centers that are only accessible by car, you have to make an explicit effort to get any exercise.