As I mentioned above, I think there are a few reasons for the mailings. While it is true that increasing the number of applicants helps with selectivity rankings, it is also true that elite colleges talk a lot about socioeconomic diversity, and mailings are one way to achieve that.
Our own high school counselor recommended our state U to all the kids, and didn’t seem to realize that the financial aid at, say, Harvard, made it cheaper… I am assuming that some of the mailings mention financial aid, or at least I hope so.
Elite college admissions officers seek students from diverse backgrounds. If the student body is comprised only of those students who knew about the school and were raised and prepared for admission, then the college class would not have the now desired mix.
Less selective schools also have a financial need to market, to attract students simply to keep up enrollment. I have also seen reps from some schools recruiting from local community colleges. Some smaller privates are in danger of closing.
So, yeah, it’s complicated. The main message is that these mailings aren’t some kind of personal invitation and don’t really mean anything in terms of chances of admission.