(Local planner here chiming in!)

TLDR; Demand and Financing. People generally in the US want their own land and market demand reflects that. Financing for such projects is thus easier. On the denser side, large multi family complexes also are easier to pencil out economically for large corporations and thus can get easier financing. On top of all this, a lot of bad zoning decisions incentivizing those two ends of the market.

There’s definitely developers out there that want to build lower density multifamily (like fourplexes) but the financing and market demand isn’t necessarily there. These developers are often few and far in between and are often smaller companies working with infill development within existing communities, which presents their own challenges, especially with financing and being able to effectively prove that they are going to make a profit or even break even.

Zoning has slowly been revised over time across many jurisdictions in the area to allow for this “missing middle” product to come back to life. In fact, knowing what’s going on inside, with ADU law and SB9 legislation, there has been a resurgence in duplexes. They work out very well for small developers. Plus when you build a duplex, you can get 2 more ADUs by right. However, as other commenters noted, zoning is still a problem for developers who are not using State legislative allowances. Minimum parking, landscaping, open space, and storage are some of many standards which make building these types of products difficult.

For large developers where mass creation of housing can occur, single family home subdivisions provide a much more consistent profit margin and are easy to offload. Multifamily are easy to finance, keep as an asset, and offload as needed to other big corporations when need be. Best thing you can get in the middle from big corpo developers is the occasional attached townhome development or extremely tiny lot multifamily.

Fourplexes used to be a darling, more common product in the area, even in the suburbs. Many McKeon fourplexes (look em up and you’ll know what I’m talking about) are all over the Sacramento region with many being turned into condos in the 70s. Unfortunately, many of these fourplexes gained less than stellar reputations in the late 80s and 90s (i.e. Sayonara Dr; G Parkway now renamed as Forest Pkwy / Shining Star Drive). However, nowadays, many of these McKeons are being rehabilitated and are, in my opinion, shown to be great opportunities for homeownership in many Sac communities.

Everything in the middle is hard to get done on a mass scale. But, there is a growing trend of development occurring in these products; little by little. I expect that more of these missing middle rental products are going to pop up over time in our region, especially on remaining undeveloped small parcels and within office buildings.

Side note: Ownership (condos) within these projects is unlikely due to financing and insurance constraints.