Google isn't making arbitrary decisions here - it's reflecting social norms. Americans aren't big on walking. I've seen pictures of beautiful leafy suburbs that don't even have sidewalks.
This is overstated. At least as of 2017, Americans walked about 93% as much as people in France, for example, and a little more than people in Portugal or Greece.
I don't have sidewalks in front of my house but our neighbors all take walks - and walk to main street. A quiet street that sees a few cars an hour doesn't need a sidewalk to be walkable.
The other scenario of no sidewalk are like rural roads that are very long with nothing in between. So there sorta "no place to walk to".
