Cars.

Everything is designed around cars. Cars require lots of space, particularly at the destination (parking). So everything is built spread out (sprawl). Now you can choose between living in a tiny apartment or something bigger, but far away from everything. When you are younger, enough of your friends might choose the apartment and you can live near friends. This is for many what makes the college experience special. As you grow older, more will choose (perhaps including yourself!) to live far away. One day you realise that you have no proximate friends.

Cars, man.