This is the arbitrage opportunity for lower-ability legacy students. If the schools provided elite educations, they would chew up and spit out lazy rich kids (and probably generate parental acrimony toward the schools in the process, weakening the donor connections.) As affirmative action of all shapes and sizes creates an ever-expanding group of people most likely to fail who the school is particularly determined to not flunk, there is more safe space for deadweight rich kids (which, for the record, I suspect is an overplayed trope relative to actual prevalance, though probably not hard to find at top-ranked schools.)