The American idea of a perfect life is to have a nice house with a huge yard that the kids can frolic/play in. A big house with a big garage. And of course in a nice school district.
Basically its supposed to be an ideal environment for raising kids. Which is weird because me and all my friends grew up in heavily urbanized areas living in apartments (outside the US) and all turned out OK (well almost all).
It probably also has to do with the independence of not having others live across the wall.