It's just not necessary. When they still had a "writing" portion of the SAT, you could buy a 700-800 by writing an essay ahead of time and adapting it to the prompt. If you had the means to afford a 'test prep' service that would tell you that, you would be head-and-shoulders above the competition without blatantly breaking the law.
And that's just one example; the entire system is tilted to make sure that affluent kids get into good schools and stay there until they graduate. You might not get into your 'dream school', but you will get into a world-class institution and have countless barriers swept aside for you over the following several years. It will tear the child to shreds in adulthood once they realize what happened, but that doesn't seem to matter to most wealthy parents.
Plus, people entering their 20s don't usually know what will fulfill their lives, what they are good at, how they can contribute to others, their responsibilities as citizens, etc. I'd like to see a re-thinking of our entire educational system, with a focus on lifelong learning.
What would be wrong with a cultural tradition of encouraging people to go back to school every decade or two? It seems like a huge waste of human capitol not to.