It is blazingly obvious to everyone outside of North American that their health woes are down to a reliance on cars, cities built to accommodate cars and an overconsumption of sugar.
It is incredible that they haven't managed to figure this out yet.
As adestefan observes, the problem is not confined to America nowadays either.
I go away and do something else instead.
Living downtown would cost over double my mortgage and significantly reduce my quality of life. I have access to trails and silent nights by living 14-20 minutes from work depending on traffic.
I also eat real foods and avoid most processed food and sugar. When I have a sugar craving, I buy some fruit.
If "walking" is your only form of exercise, you may curb obesity, but you'll still be far from perfect health.
The human body is an incredible contraption, and there is almost always room for improvement and growth. Get stronger, faster, and more durable.