First of all, it costs too much to build real estate. Cities need to constantly reshape themselves to fit changing populations, energy costs, and work opportunities. Adaptation is crucial.
Second, they're too big. It's crazy to require people to have a car to get around. Walking, biking, and public transportation fit a considerable segment of our people (the young, the old, the poor) and shouldn't be so hard to use.
Third, I'd like more opportunity to inter-act with others. If I live on a quarter acre ranch, I'm not inter-acting with my neighbors.
So I like this idea I noticed:
http://www.jetsongreen.com/2014/10/a-shipping-container-apartment-complex-soon-in-dc.html