https://hechingerreport.org/proof-points-why-elite-colleges-...
"To find out why elite colleges love legacies, two business school professors were granted access to 16 years of admissions data at one elite Northeastern college. The upshot: it’s in this school’s clear self-interest to take them. Alumni children who received offers matriculated at much higher rates, giving the school more certainty in their future enrollment numbers. And these loyal families with multi-generational ties to the college were far more likely to donate funds, money that the school needs, in part, to offer scholarships to others."
Colleges want to look good... but no chance in hell they'll give up consistent students of families who donate.
Cost effective? not when it'll cost them millions or billions in kick backs.