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.