It was a thing that grabbed people’s attention. It still grabs attention. (Perhaps one reason why Stanford stopped publishing their acceptance rate?)
What’s more interesting to most people?
“Harvard accepts only 4% of applicants!”
Or…
“Most students graduate from Harvard in 4 years!”
Everybody wants to be in the club that’s hardest to get into. Watch any college decision reaction video. Like it or not, people care about it. It doesn’t matter if acceptance rate was only 2.5% of the ranking criteria.