>the only difference between now and then is we have far, far more regulations and zoning rules in the way.

Some places have more or less run out of land. For example Vancouver is limited in it's north, west and south expansion by mountains, ocean, and an international border respectively. I believe there are similar issues in San Francisco, Seattle, New York, etc.

Indeed, higher density is the answer... I'm just pointing out that the increase in regulations is not the "only" thing that's changed.