Others have pointed out, but I will emphasize, that this runs smack into the local politics of suburbia. Most people don't want to live next to a multi-unit house, and so: 1) the value of homes around a multi-unit house will decrease, causing enmity and dislike between neighbors, and 2) neighbors will respond by forming HOAs that restrict the number of units allowed in a house. My own HOA, for example, completely bans renting out my entire house, much less subdivided portions thereof.

Reading this makes me so glad I live in a part of the country where there are no HOAs and where multi-unit houses (2 and 3 unit freestanding) are typical. It's a great neighborhood, and the high density means I can live within walkind distance of way more friends than in the part of the city where I grew up.

Why in god's name do people have such a problem with living near multi-unit housing?

Well, there's street parking, for one. In SF, for example, developers are not allowed to put as many parking spots as there are units. This is supposed to drive people to use mass transit, but more obviously increases demand for street parking. OTOH, single-family housing almost always has dedicated driveways/garages to eliminate this issue.

It's all about profit for the builder in relationship to municipalities and their zoning codes. Municipalities that were mostly formed around the time of the suburban building boom instituted very strict zoning and regulations around density, subdividing properties, adding in-law units etc.

Fast forward to today and in all but a few very progressive communities, the fear of changing these zoning laws negatively impacting property value has essentially frozen them in time. So in a lot of suburban areas you couldn't build separate living units on one single family lot, even if you wanted to. Or at least it would be quite costly.

For instance my brother-in-law is a planner in a suburban California city, discussed this once with him... apparently in his city you need to add 2 additional parking spaces for every extra housing unit added to your property. Want to make an in-law unit for grandma? 2 parking spots please. Don't have room for an extra driveway? No permit.

Basically unless new construction was zoned to be a higher-density area, for apartments or townhomes etc, for a developer it doesn't make much sense to build with extra units in mind. Especially if the builder has to put up a fight for permits or spend that much more money to meet code. In that case, it's more profitable to just be building condos.

Christopher Alexander talked about that in A Pattern Language, and the New Urbanist architects talk about/design for/build it quite often (the usual solution is the "carriage house" or "granny flat" but more generally the idea is to restrict zoning to address patterns of activity in public spaces, thus legalizing more variation in demographics and behaviors across time and short distances). It does not really fit into the mainstream (suburban) scheme of fastidious homogenization of uses and forms.

That would be called in duplex in most cases.

I think they don't because then you get into multi dwelling zoning and if the property was zoned for single-family homes it could get complicated (probably not by much).