As a father of three in a a suburb of a major northeastern metropolitan area, I think it is due to two things: media and moms, and the feedback loop between them. Almost no children walk to school around here. Almost no children bike to school, or events, or just go out and play and run around by themselves during the day. Their lives are scheduled and they are shuttled from place to place by harried parents as necessary. The reasons are an unjustified fear of danger. As a father and husband, my success rate in arguing against these fears using statistics and logic is near zero. No, wait... it's zero.