Because it really doesn't. The total incoming undergrad class is ~1650. Across the entire Ivy League it's around 17000. If 20% of admissiosn are affected by legacy/faculty/donors, then that's like 3400 people who wanted to go to the Ivy League who couldn't (I suspect this is a high number, but ignore that). To include all the other higher tier schools ehre this might matter, let's just call it 10,000 people affected. There are 4.3M 18 year olds in the US. This means that roughly 0.2% of people in an academic cohort will have to accept going to a slightly lower tier school then they would have otherwise due to legacy admissions.

BTW- There are a fair number of studies out there suggesting that students who are admitted to top-tier schools but don't go have no effect on longer term outcomes than those who do, so other than short term disappointment it might be a null effect.

Meanwhile, universal pre-k enables families (ane espicially mothers) the ability to return to their own work/careers faster and has been shown to improve long term academic success (although their is some question to scalability).

So between the two, universial pre-k seems like a far more important issue...