if people still use the street instead?

So does anybody else see people walking, running in the street when there's a perfectly good, level, unobstructed (for blocks) sidewalk less than 3 feet away from them? Anybody have any idea why? I've seen walkers, runners, people pushing baby strollers in the street in our subdivision even though the sidewalk right next to them has no cars parked in the driveways blocking it, no tree roots making the sidewalk unstable, etc. Just one of the many ways that I cannot fathom what people are thinking when they do something that just seems so absolutely illogical, but would love to know if anybody knows of a reason for this. I guess I should just be glad that they're going against the traffic the way they should be. Probably have to pull over someday next to one of them and just ask them why (and then hope I understand the reasoning behind it). People in the other places I've lived used sidewalks if they were there, I don't think I recall a case of someone ever using the street if there was a sidewalk, which is why I'm asking.

So should we keep building sidewalks, even though people don't use them?