I don't think that's true. My grandparents live near a town of 4k people or so that's not walkable in the slightest, spread out over miles of highway with massive city-sized streets criscrossing everything. Certainly nobody can walk to the grocery store, which is on a hill far away from the residential areas.

I'm sure they exist, but what is around the grocery store that there's no houses within 2 miles?

Often the "new developments" near these towns are out of town a bit, but even two miles isn't an insane walk.

I'll grant that many of them don't have much in the way of sidewalks, but you can often find them.

It has some unbuildable hills on one side, the county hospital on another, and then parking lots plus a giant highway interchange filling out the rest of the space.

Go visit Chadron, Nebraska sometime. On foot. You'll see the error in your generalization.

Arial reconnaissance seems to show it similar to the town I'm in; it could use a bit more sidewalks but the old downtown is certainly walkable and even Walmart shows obvious paths where people are walking across the grass/ground.