The land, foundation, and utilities (often in the form of impact fees) are largely fixed costs regardless of home size. The larger the home one builds the less impact they have on the final cost to build. This is why “starter homes” don’t get built anymore. The builder has every incentive to build that 3,000 sq ft home on a small lot instead of something more reasonable in size.
Also, other options that used to be more efficient to build multiple homes at once, like row houses or duplexes/triplexes, are generally directly or indirectly illegal to build in most US cities now. Row houses and brownstones were the cheap-and-fast housing option once upon a time; they're now rare and expensive because in most places nobody's allowed to put up a new one.
I've seen sixplexes around here, but unless they're apartment buildings (which have better designs anyway) they're relatively undesirable.
A duplex saves you some coin, but each additional house you stick on the side saves you less, and the desirability goes down. People don't like duplex/triplex/sixplex living.
