There is actually a very good reason, namely that acquiring knowledge, skills, wisdom, etc. is at odds with acquiring assessment signals. The only way to become a good entrepreneur is to basically become unemployable.
I would disagree that the only way to become a good entrepreneur is to become unemployable. The best way is, as you say, to have knowledge and skill to create a useful product. Then, having a market that allows you to easily get paid for your work helps.
I mentioned high school specifically because I know that I would have loved something interesting to work on. It's a perfect time to work on skills rather than assessment signals. Lots of kids don't even bother with the assessment signals...
Sure. Once you've actually created a product that's useful, or at least impressive from a technical and/or design perspective, then you're easily employable at that point. But for most people there is still going to be a several year gap between when they start focusing on acquiring those skills and when they've actually built something impressive enough to function as its own signal.
Maybe in the next 10-15 years we can convince high schoolers that if they spent their free time programming they could be capable of creating a revenue generating asset at 18.
The combined power of a nation of kids that could create rails SaaS sites or reusable 3D models would be an awesome sight to behold. It's a skill they would keep and even pass on.
Tech is largely new, and we really haven't bothered to teach it yet.