Legacy admissions is a different issue (though that's a problem too).
It is not a well publicized fact that 'people who donate get even higher preference', even calling it 'preference' is a euphemistic way to obscure the bribery.
It isn't preference, there's some hidden price and people are paying it for a slot.
Nobody is paying for a building and finding out their kid is not accepted.
Show me where on any top university website it says 'as an alternative to our standard merit-based admissions process you can contact this number to discuss payment for admission'.
They can't say that, both because it would reduce their leverage in negotiating the bribe value, and because what people are paying for (in large part) is the social status of having been accepted to an exclusive institution on merit.
Universities know this, it's why they spend effort encouraging applicants they know they will never accept in an effort to drive down the acceptance percentage and drive up their exclusivity.
If it's public that your rich parent bought your slot then suddenly a lot of that status evaporates.
Since the social status is more the product in question than the actual education people paying for it don't want this to happen.
This being public is bad for the school's brand too since knowledge that a lot of students can buy their way in would reduce the status that they're selling in the first place.