The obesity in America has very little at all to do with walking or not walking. It has everything to do with the American diet.

Fine, then. I have four words for you:

Drive-though fast food.

(And if that’s not pointed enough, here’s a video about how much easier it is to shop for healthy groceries in a walkable area, and another that points out (among other things) how lack of walkability correlates with obesity even if, as you say, it’s not the single direct cause.)