There’s so much unincorporated land in the USA. So many opportunities to prove things work or don’t work. Why move into a place somebody else built?
Mostly, probably, because starting a city has a process defined in state law (or, out of the US, law of some other higher level of government) which is different than “a bunch of people just buy up land and decide to declare it a city”, e.g., California’s LAFCO process.