Because there are so many applicants that have good grades.
A more cynical view is that the governing boards want a way to pick and choose who they let in. So they create "holistic" application systems to get "360 degree view of the candidate".
> Because there are so many applicants that have good grades.
So train more doctors.
> Because there are so many applicants that have good grades.
Sounds like we need more spots for these people to go
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.