> What is stopping them exactly?
Living in places not suited for it. I don't have a drivers license. I'm 48 and have never gotten one because I like to walk and have always lived places with good public transit.
So, when in a past job I often travelled to the Bay Area, I tried to walk as much as I could there too, and it often worked.
But I also frequently found direct routes to places I needed to get to unsafe to walk, and ended up on massive detours. E.g. once while we had offices near Menlo Park, I stayed in a nice B&B in Atherton. The direct route along El Camino Real would've been short enough to walk (for me anyway), but there are parts of it that have no pedestrian affordances whatsoever and woefully insufficient lighting to walking along the roadside. I did that. Once. The detour I found (there might well be better ones; I tried once and wasn't very familiar with the area) took 2-3 times as long at the time.
Had the direct route "worked", I'd have loved to stay at that B&B again on future trips, instead it was written off as too inconvenient for me.
As a visitor, I took some perverse pleasure in trying to figure out how to manage there without a car. But had I lived in the area, I'd probably quickly have given up and resorted to learning to drive.
And frankly, that's one of the more pedestrian friendly areas I've visited in the US outside the highest density urban cores.