> If just you want to test their ability to perform in school, why aren't grades the better metric?
Because grades are not independently assessed by a third party? No matter what the theory behind it, the SAT is really, really close to an independent test of learned material. And many people would surely argue that having your prerequisites down pat and being fully ready for subsequent learning is at least a major part of what "aptitude" means in practice.
At the point where you’re entering college, how well you’ve mastered and can apply the basic math that SAT tests basically is your mathematical aptitude in the “suitability or fitness” or “likely to succeed at college math/engineering/science/logic” sense of the word.
Because grades are not independently assessed by a third party? No matter what the theory behind it, the SAT is really, really close to an independent test of learned material. And many people would surely argue that having your prerequisites down pat and being fully ready for subsequent learning is at least a major part of what "aptitude" means in practice.