If they are an incorporated plece, and they get the whole shebang of taxes, funding, elections, etc., then they need someone to run it, or a “government”.
Guess it sometimes could be hard to tell, with all the varying rules, but people who live there… are there places in the “deep US”, where people live in anarchy?
(sometimes it might also be the case where an incorporated town, through depopulation, ends up with nobody willing to act as mayor, but that’s different)