Most teenagers have no idea what they want to do with their lives and a lot that think they do end up changing their minds. And if you only learn things relevant to your expected career, you will be fucked if you want to do something different.
Lol, if we did that, humans would probably be even more fucking stupid. We should attain as much knowledge as we can since it might be needed in the future. Horrible idea.
That's what college is for. You pick a major that will give you the basis for the career you want to pursue.
If that happened, our society would slowly turn into mindless workers. Sort've like what happened with the ants in Antz (the poor man's Bug's Life).