High stakes entry-exams have their own problems. Among many other issues, they measure peak performance, rather than sustained performance.
US college admissions have many problems, so don't hear me say they don't need reform, but you are also assuming that academics are the sole thing that college is about. It isn't, at least not in the US. US schools value many things beyond pure academics. Among them being leadership (whether or not you were class president is one such indicator), personal initiative, success against the odds and many other things.