It would be a lot easier and more marketable to take the more run down single family homes that have become too expensive for most people and do rebuilds that turn that lot into two homes. The financial model for a developer is easier too, you do the transaction and you’re done, as opposed to needing to be a landlord for a long time, or find just the right kind of investor to buy a 4-unit apartment (which is really rare).

There is an entire spectrum of housing types that are generally referred to as the “missing middle,” which are common in old cities and European cities but virtually non existent in post-ww2 America.

That’s what removing or relaxing single family zoning should allow.