The SAT used to be marketed as an “aptitude” (what the A in SAT used to mean decades ago) test. I.e. an IQ test. However, like other IQ tests, it is still influenced by environmental factors (including schooling and coaching – high school English teachers would make learning SAT vocabulary words a weekly part of English classes, for example). Now, it is not so much marketed as such (A was changed to “assessment”, then more recently SAT is just SAT, not “Scholastic A______ Test”), though it is still widely seen as such, since it is much less focused on specific content knowledge than other tests like the SAT subject (formerly Achievement) tests, AP tests, and many other standardized tests.
Many studies do show that HS GPA is a better predictor of college GPA than SAT scores, but the likely reason SAT scores are still used is that high schools in the US are not very consistent with course content and grading policies.