No matter how many have good grades, you can always pick the top n by grades—unless there's a ceiling that the top m > n have all hit. Which, if you're talking about "grades" as in GPA, is plausible.
MCAT seems more relevant, though. According to Claude: "Roughly 0.1% or fewer of test-takers score a perfect 528 in any given year — typically only a few dozen individuals out of the ~120,000 or so who sit for the exam annually." So it should work fairly well for them to sort by MCAT and take however many they have (or expect to have) room for.
I think OP's point was that the governing boards don't want the people with the top n grades. They want certain people, and by making the admissions criteria fuzzy, they can pick and choose those certain people and then say "well, our admission criteria is subjective," and "we are looking for 'well rounded people," and all kinds of other vague weasely ways to let them legitimately shape the student body in the way they want.
See also: "Cultural fit" when hiring.
At a certain point, grades become arbitrary and won’t necessarily select for the best candidates. Obviously the current system doesn’t, either.
The actual solution is to increase the number of slots for training doctors to match the huge number of qualified applicants. It makes even more sense given that there is a shortage of doctors and health care costs are astronomical.