Like most barriers to wealth and home-ownership, I expect it has mainly to do with the US economy making it really hard for people to accumulate enough capital to do so. But also maybe because they have a sense of how much time and work it would be and how many people need to be involved, and bureaucratic obstacles, and probably many other details I don’t know about. Of course, there’s also at least one cooperative that does organize people to build homes for other people in need. And some people involved in home building do end up doing at least some of the work on their own homes.