At the levels we’re talking about, the difference between “perfect score” and “still excellent but imperfect score” can be down to one or two errors on the test, basically noise. Given the number of applicants and the willingness to drill testing skills, going by scores would eliminate anyone who makes a minor error. The ability to perform perfectly in high-stress situations is a useful (and learnable) skill - like the ability to win quiz games, or perform classical music at concert level - but it is not the same thing as academic potential.

(As an aside: as an advisor of PhD students in computer science, I sometimes have to train students out of these habits. Research doesn’t happen if you’re used to getting the shallow answer more quickly, or if you’re afraid to be wrong.)