In most places in America walking is not a practical mode of transportation because of distance; it’s miles from your house to the grocery store, for instance. If you live in a city this might not be obvious to you but half the country lives in the burbs or rural areas.
Separately, it would be nice if we had more sidewalks, but they are financially unsustainable in low density areas.
Separately, the US does have an obesity problem, and exercise would help, but the biggest issue by far is dietary sugar/carbohydrates.