If you give a child a fish, you feed them for a day;
If you teach a child to fish, you feed them for a lifetime;
And if you teach a child to learn, you feed them for a lifetime and they don't have to just eat fish.
But if you teach a child to think, you prepare them for a lifetime where they can do much more than eat.
The purpose of training is to teach people how to do something. And as long as there is a market for that skill, they are employable. However once that skill is no longer necessary, you need to train them to do something else. That is fine as long as they are still trainable. The challenge is that people who are inherently not curious and lack a passion for discovery will rapidly turn into that "old dog that can't learn new tricks."
The purpose of education is to teach people how to think, utilizing the vehicle of learning things. In other words, I see myself as more educated (in that I can think in a variety of ways) than I am learned (I don't remember much about what I was specifically taught in most of my subjects in primary and secondary school, college, medical school and post-graduate training in psychiatry).
For instance, I don't remember much about geometry and even less about trigonometry, but when I am working with a group of three people who are not getting along I think of how to turn their "acute" or "obtuse" relationship into one that is "equilateral."
As another example, I don't remember much about algebra, but I do remember simultaneous equations and how you can solve for x, using y and z. That has come in handy when helping individuals make breakthroughs when they are stuck in either their thinking, feeling or actions. I find that whenever one is stuck in one of these things, and keeps banging their head against the wall to get through, they can just give it a breather and throw themselves into the other two and usually that will spontaneously break the log jam in the first.
And finally, to quote Sam Cooke, I "don't know much about history," but I do remember that President Reagan broke through a conversational stalemate and the Cold War when he reached out to Russian General Secretary Gorbachev and said, "Call me Ron." In more recent times President Obama did the same with Hilary Clinton who was disinclined to accept the position of Secretary of State when he said to her: "I need someone as big as you to do this job. I need someone I don't need to worry about. I need someone I can trust implicitly, and you're that person ... You're worth it. Your country needs you. I need you." (From "Game Change" by John Heilmann and Mark Halperin).
That last example would be a good way to reach out to your "Why do I need to learn X?" child. Perhaps you could say to them, "We love you very much and our job as parents is to prepare you to succeed, be able to compete well against anyone, do what's necessary to get any job you want and to do all of these eventually without us. We need your help by your going to school and learning every and anything you can so that you can be prepared to do whatever you want to do in life and succeed."
If it worked for Reagan and Obama, it could work for you.