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.
If that were true, then there wouldn't be any shopping malls, or WalMarts, or bars, or churches, all of which can be found in any exurb.
