I've always found it puzzling why universities seek information beyond a student's academic performance. It seems odd to me. Imagine if professional sports teams had "legacy admissions" or "affirmative action"...
BTW this is also why such institutions pay so much attention to.extracurricular activities, clubs, sports, traditions of certain elaborate mischiefs, etc. These all are bonding mechanisms that make the alumni networks more tightly knit and thus more valuable to the alumni.
This is a significant reason why they are glad to accept legacy admissions: it helps keep the links between fresh graduates and influential but older alumni, again making the network more valuable.
The academic load helps keep those with weak intelligence and willpower away. It also provides useful knowledge and a formal degree, but it's sort of secondary, technical detail.
Money. Status.
The parents of legacies are… alumni. Alumni are the same people who are the biggest donors, the biggest cheerleaders (spreading the virtues of the university to people they talk to), and might even participate in the university application process. Frequently alumni will identify high talent kids and encourage them to go to their favored school. The joke that “daddy bought the new building on campus so Johnny can attend, despite low grades” is a trope, but it’s not wrong.
Affirmative action was (1) an effort to apply similar representation to the university to the wider population in the country (2) bring more diversified experience+culture+thought to campus and (3) to try and level the playing field after 200+ years of rejecting people based on things that are irrelevant to academic performance.
You seem to think that life is entirely a contest of merit. In practice, large groups of people almost never value merit over wealth, status, exclusivity.