This question is based around a basic philosophical concept. Assuming no aliens exist, if something is not perceived by humans, it may as well not exist. If I draw a card from a deck of cards but keep it face down and no one ever sees it, the card may as well be blank, and for all practical purposes, the suit of the card does not exist. On that same line of thinking, if humans will never be able to travel outside our solar system, why should I learn about how the universe is so intimidatingly large. And, if you critically look at every fact in a textbook, most of them would only be useful to someone with a very specific job. This is an apathetic look on the world, but one that is surprisingly common in school.
Most people would agree school is intended to educate children and teens with the skills they need for their life and career. But aside from basic math and reading skills, there are few things taught in schools that everyone absolutely has to know. The most necessary skills for later life that I can think of are the ones taught in world languages and home economics, and still most of America only speaks english and tons of people can't do their own laundry. Language arts is useful for writers and government for politicians, but not everyone is a writer or politician, so why should everyone be required to learn the skills required for one. My math teacher himself has said that the most practical application of learning math is getting a job as a math teacher.
But if thats true, why has it been shown that scores on vocabulary tests of words that no one uses correlates almost directly to someone's income, or that it is almost impossible to get a living in the middle class if you never learned algebra in school?
Because, although school may not be a direct preparation for every possible scenario, it does help people My math teacher from seventh grade put it best once "School doesn't always prepare you for getting a job, but it imitates life with a job. You probably won't use the formulas I'm teaching you twenty years from now, you probably won't even remember them, but the time it takes to memorize them will teach you the need to do work. For example, this summer, I had to learn how to make webpage for my class. And I wouldn't have had the work ethic to spend the hours on it had I not had to memorize similar things in school." And, all things considered, every class doesn't train you for every career, but some skills needed for some careers are covered in most classes.