Universities get substantial federal funding, including Pell and research grants. Such investments are put at risk if the universities fill themselves with unqualified applicants.
And what makes the unqualified applications requiring building purchases for admission ok, and the unqualified applicants requiring cash payments to coaches not ok under your rubric?
Should be patently obvious that there’s a cap on how many people can donate the kind of money that funds a building — nevermind that such donations are public. There is no cap for a secretive system of bribery and arbitrage.
Because donating money directly to the school benefits other students who use the dorms, the building, the research facilities. Straight up bribing the coaches just enriches the coaches.