I would contest the idea that this construction isn't happening - it's largely just resulting in sprawl at the periphery of existing cities. America has a lot of space available and a lot of space actively being developed.
As to why new cities aren't being built in completely empty areas, I would have to know where you're talking about to make a guess as to why they aren't being developed. Off the top of my head:
1) Environmental concerns
2) Expropriation concerns making projects politically untenable
3) The concerns of aboriginals
4) Cost of infrastructure development
5) Lack of market demand
To elaborate on the fifth point, I would posit that people don't tend to populate such greenfield cities unless there is a compelling reason to do so; either by being pulled (e.g. by resource extraction opportunities, a growing economy, educational opportunities, etc.) or by being pushed (e.g. fleeing a war, the effects of climate change, or political persecution). China is the example to look at here. They spent most of the 2010s buying up an enormous amount of resources to build cities that ended up just sitting empty before eventually being demolished. A lot of this was just fraud (the buildings weren't constructed to be habitable to begin with), but at least some proportion of it had to do with the factors I mentioned above. The cost of physically moving to a new city, as well as the loss of social capital resulting from such a move make relocating to a new city both undesirable and prohibitively expensive for most people.
I do (becuase I've heard it from a buddy of mine who's an MD at a Bulge Bank) - there is no capital to invest in consumer real estate anymore.
Large portions of that industry died out in 2008-12, and capital for large real estate projects like a housing community tend to be allocated 2-3 years before ground breaking, and then an additional 1-2 years to build.
So to build a brand new community by 2020, you should have done all the leg work in 2014-16. And to build a new one today, it should have been done in 2019-20.
Any pipeline that even existed is now dead, because manufacturing construction was the asset class of choice in the 2022-24 period along with high interest rates (making projects much more expensive) and tariffs on Canadian lumber, so the housing shortage is about to get even worse.
As to why new cities aren't being built in completely empty areas, I would have to know where you're talking about to make a guess as to why they aren't being developed. Off the top of my head:
1) Environmental concerns 2) Expropriation concerns making projects politically untenable 3) The concerns of aboriginals 4) Cost of infrastructure development 5) Lack of market demand
To elaborate on the fifth point, I would posit that people don't tend to populate such greenfield cities unless there is a compelling reason to do so; either by being pulled (e.g. by resource extraction opportunities, a growing economy, educational opportunities, etc.) or by being pushed (e.g. fleeing a war, the effects of climate change, or political persecution). China is the example to look at here. They spent most of the 2010s buying up an enormous amount of resources to build cities that ended up just sitting empty before eventually being demolished. A lot of this was just fraud (the buildings weren't constructed to be habitable to begin with), but at least some proportion of it had to do with the factors I mentioned above. The cost of physically moving to a new city, as well as the loss of social capital resulting from such a move make relocating to a new city both undesirable and prohibitively expensive for most people.
Do you have another theory?