One of the problems is that there are actually relatively few big cities in the U.S. that also have the quality of being nice places to live. Most of them are huge (or formerly huge) manufacturing centers, populated by people looking for unskilled jobs. Cities are generally built as "nice places to live", they arise because cities are efficient nodes for economic exchanges to take place. Some urban areas have almost no housing at all! http://en.wikipedia.org/wiki/Tysons_Corner,_Virginia

Outside of parts of New York, SF, Seattle (and maybe 2 or 3 others), most urban centers are high crime dumps with the additional quality of being frighteningly expensive to live in and then completely surrounded by vast heavy industrial wastelands.

Don't get me wrong, I'd give my left leg to live in NYC at the standard of living I have in boring suburbia. But back of the envelope calculations show I'd have to make 3-7x what I make now.

At least the nice folks that built my suburb designed it with a walkable layout to a commercial town-center stocked with everything from a Gym and Movie theater to a dozen restaurants and groceries so I can pretend I live there.

Most people who write this kind of stuff really don't mean "people should live in cities"...what they really mean is "people should live in city (singular), and that city should be NYC or SF (or Seattle in some cases)".