It's a population problem: There's only so much good real estate.
The bigger the population, the fewer of us get to live on it.
There's so much land we could quadruple the population and not even notice it. Have you not ever looked out the window of an airplane. Most of this country is unused forest.
Thats debatable.
I'll concede, I am biased, I work in real estate (In NYC no less).
As has been pointed out, and numerious times (beyond repetition and almost annoyingly so) you could fit the entire population of the earth (over six billion no less) into an area around the size of texas (if you gave each individual person, man, woman and child indiviudally, around 1,1500 feet to live on or as we call it in NYC, a single bedroom apt, bare in mind, the "texas model" assumes no 2nd floor)
In other words, RE wise, over population is a myth, the entire population of the planet, if it so chose, could live in housing in a area the size of a large US state, without going overboard (over that states lines) sinlge story wise and be just as good to go as now (or better).