Purelly and entirely because when it comes to money with no strings attached, having X + A money (if A is a positive real number) always beats having only X money, hence why lots of people are pointing out Universities with X in the billions of $ which still ask for donations.

It's pure unadulterated greed, trying to monetise a (not anymore deserved for most in the US) public image of Universities as places that help people have a better future.