They really aren't. Sufficiently slow traffic (or none at all) will render streets walkable. Prioritizing pedestrian use over vehicle use is what creates walkability.

In the US you tend to find this in places that were built before cars but with sufficiently high property values that rearranging the buildings to widen the streets doesn't make sense. See parts of Boston for example. The places that are both old and wealthy tend to have streets that are pretty much local access only through size alone.

In a dense enough place with narrow streets you can just walk on the road. A lot of old towns are like this.

In montreal there is an extensive system of back lanes, and I did a lot of my walking there, no sidewalks. The lanes were shared by cars but they were slow and infrequent.

Residential neighborhoods in the Tokyo metro area are generally walkable but have very few sidewalks.