Kilgore Trout
Misanthropic Humanist
To me but not the cities but how the population is grouped. Half of the US population lives in less than a 10th of the area of the US the other half lives in 90% of the area of the US.
The evolution and development of human civilization tends towards a higher percentage of the population living in cities - the more developed the nation, the more pronounced this effect. Also, for anyone who has traveled extensively around the US, it is plainly visible that enormous swaths of the country are not habitable, are not desirable to live in, or not able to support large population centers.