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.