Because our culture is lurching towards paranoia and isolation. People are afraid of other people, big groups of people, and big buildings out of a fear of other people. People have come to believe that they will be victimized by criminals or terrorists if they spend too much time out in the world. It's a part of our culture now and you can see that in a lot of aspects of TV and movies. Most sitcoms take place IN the house now rather than out in the town. The city sitcom trend of the '90s is pretty much over and now (surprise) people are watching shows about terrorism and murder instead. The people that are watching those shows and scaring themselves shitless about the world are also the libertarian "back off" types because of that fear and paranoia.
Of course there are going to be
some, and mostly ones that require as little interaction as possible like WalMart. Like what opebo was saying though, the streets are largely empty and communal gathering places are nonexistent. There's no desire among people in general to connect to others in their community because those communities are being replaced by cookie cutter developments with cookie cutter families in them, all of which could be any given person in any given area. Everywhere in suburban America is turning into the same town.
