I think that cause and effect might be backwards - people only ever lay out the large upfront expense to build density when the land becomes valuable enough. And that only happens when people are clamoring to live in the same area.
If you then refuse to build up, it makes the affordability much, much worse.
> If you then refuse to build up, it makes the affordability much, much worse.
How? If you've balanced jobs/wages against housing supply via standard planning practices, then you should be able to tweak demand.
If demand is still high, density just drives prices higher, and shifts the entire housing economy to a rent-seeking one.
Furthermore, the real-estate market is global, but the housing market is local. My parents own homes in another state that they've never even seen, and merely rent out via a property management company.
As land prices get driven up through density, how do people participating in the local housing market compete with those in the real estate market?
If you then refuse to build up, it makes the affordability much, much worse.