Because cities are where success is. People who are already rich like living in the countryside. People who would like to become rich need to live in cities.Huh? In the US, median rural income was $40,135, compared to $51,522 in metropolitan counties. [1]
Things that are for sale, are generally for sale in cities. Want to buy something? Your best bet is a city.
Please tell me how long the drive from SF to the nearest Walmart is.
Cities aggregate enough people that you won't be the only person like you.
This is a very good point. Lots of interest-based communities form in cities. In suburbs, there would not be a sufficient "critical mass" for this kind of community to exist. It drives business too - there are, for example, restaurants in cities serving niche foods. They would never have a large enough customer based outside of a highly-concentrated urban area.
[1] http://www.ruraledu.org/articles.php?id=3086