@thumper1 I’m 100% sure what I’m saying is true. I think most don’t do it because while it is much easier to gain residency in Missouri than other states, it still does require a good amount of time and effort. Most college students wouldn’t really want to stay for the whole summer, they’d rather go home for the summer, so that probably deters a good number of people. It is pretty popular among OOS students though because they have a lot of information about it on their website, plus, like I said, they basically encourage it at information sessions.
There’s a guide here that explains it, and it’s pretty much a 6 step process that is very cut and dry: http://admissions.missouri.edu/costs-and-aid/residency-requirements/documents-and-steps.php. Everything I’ve said here to the OP is also stated in that guide.