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.
Because you were a child.
In all seriousness, because if you dropped these same people in the woods with construction tools and resources, they’d produce nothing of value; all eaten by bears within weeks.
IOW, your question answers itself.
I don't understand unincorporated land, but clearly someone "owns" it or the government wants it for some reason and the former era of free expansion and manifest destiny is over. Now we must reckon with the damage and reparations owed to Native Americans.