The other side of this is people like Chad Hall or Lucas Verzbicas who got recruited to Oregon as freshman, then didn't finish their eligibility there. I'm sure there are many others, but these two are well-known. Chad Hall transferred to UC Riverside (partially to be closer to home), and Lucas wanted to pursue the triathlon as a pro (after running a sub-4 minute mile in HS). If running was still the top priority for them, they would likely have been better off to stay at Oregon.
A big part of it may be a runner developing into a national class runner at their first college and realizing they would have better opportunities to train and compete at Oregon. Matt Centrowitz (senior) transferred there after his first year at Manhattan.
Good point that hosting big meets (including the NCAA championships) equates to free recruiting/advertising. Other schools do this too -- especially with hosting high school races. The Stanford XC invitational gets > 1000 high school kids on campus.
There's got to be a point of diminishing return for the coaches also to have too many athletes to deal with and limited scholarships and limited walk-on spaces on the teams.