For one - all the easy places to build now have things in the way. Because they had things built there.
In the 50’s these were mostly empty range land, or timberland, or farms, or swamp, or whatever. One (or just a few) owners, low property values, little to no political/environmental resistance - relatively easy to buy someone out and develop.
Now? HOAs, tons of urban zoning rules and environmental rules, thousands of stakeholders for any sizable project.
Which also now means more stakeholders, more political resistance, more BS if you want to increase density there. Or building in not-so-easy places, like steep hillsides, more remote locations, etc.
Most of the existing folks don’t want change. In 1950, especially out west, there essentially were no ‘existing folks’. Even the natives had been wiped out.
And as an economy, we were used to building things at a war economy pace, and had reasons to continue to want to do so.
Now everyone would rather blame someone else (or ensure they get their sizable cut) rather than do anything about it.
As the boomers slowly die out and assets change hands, the momentum will change. Eventually. But we’re talking about a decade plus.
Then it will be the millennials turn to be the ‘bad guys’.