Suburbs that people don’t leave are just small towns.
Suburbs sometimes have a town center but often it's much more amorphous with different strip malls and such. Everything is spread out. There is no place where you're at the heart of the town, in the center of activity. And even if there is such a place, it's probably driving distance, just like everything else. You need a car because everything is spread out because all the areas of activity are surrounded by parking lots.
The exceptions are towns with a hospital or college that can retain some vitality in the downtown area.