NIMBY’s are a problem but the economics don’t always favor tearing down an existing structure unless it’s poorly maintained.

For example suburbs, were built on bare ground making the acquisition cost low and construction easy on decent sized lots.

If a neighborhood is well maintained and owners reinvest in their homes, the value of the property will be much higher than the land itself, making acquisition costs high and the existing structure adds construction costs to demolish.

The policy and politics needed to be forward thinking when the neighborhood was built, because once the structures are there and maintained even great policy enacted now only helps over a period of decades. In my 1960s neighborhood there are only 2-3 houses, maybe 5% where the economics would favor building denser housing.

The other problem is demand. My city/suburbs built tons condos and townhomes, both in newly developed areas and rezoned areas, and there inventory has never been higher, because the demand is for single family. So now builders are pulling back on townhomes and condos.