You're right that a perfect SAT isn't a guarantee of success, although anyone who nails the SAT will be admitted to a pretty good school (assuming their GPA is commensurate).
Part of the issue is that the SAT is not difficult enough to stratify the applicant population, which means that it's not a super useful tool for schools at the very top. The elite liberal arts college I attended liked to brag that they turned away half of the 1600s that applied — and their overall acceptance rate is much higher than H/Y/S.
It would be interesting to think about what would happen if the SAT were more rigorous (or if there were another test that students could take that offered this stratification). Some schools (think Caltech) would probably put a decent amount of weight on such a test. Other schools, who currently optimize for more variables, would presumably not pay much attention.
