When students from high school know what they want to do in the future, why not go to study for that job straightaway? For example, if you want to become a doctor/nurse, then why not go and start studying about medicine? And for those who want to become architects then why not go and study all about it (math, sketching etc)?
My first reaction is just "What's stopping you?" Some high schoolers do study more about the body and other medicine related topics in their spare time. Some spend a lot of time sketching and read about architecture. I did programming in my spare time in high school and ended up a software developer. And that pre-existing experience and knowledge helped immensely when I actually got to Uni and studied it formally - I do believe I got a better education because I cared about it and because I was prepared to get the best out of it.
But I think what you are asking for is not a hobby as an extra, but removing some other parts of the existing formal education and replacing them. I think that's more of a problem. Take me: I'm a fairly straight maths-science student. I hold a software engineering degree, and work with co-workers who almost view bad spelling (not a software dev strong point) as a badge of honour. If I could have done less humanities and more math/science or software I probably would have. And yet I'm pretty sure I've referenced the humanities part of my Year 12 more recently than I've referenced the maths or the physics.
Those are just random factoids, but there's a bigger picture. As Sine says, life is about more than just what you are qualified for and what you work in. But so too is the workplace. One important thing frequently talked about is "soft skills". You can be technically brilliant and still not achieve much if you need to work with others and can't. Also, different people have different areas of knowledge, different things they know and don't know, and make connections between different areas of knowledge, both in work and in life. An overly narrow speciality doesn't help with that.
I'd suggest reading this:
Dr. Fauci Has a Classics Degree!Finally, people take time to mature, and people change. Some people have a dream career from relatively early on and stick to it (I'm actually one of them). Others find that it's not what they expected, or that they develop new interests, or that they really need to do something very different. That will be harder to navigate if they have committed to that path earlier and have fewer other options.
My parents encouraged me to focus on getting a vocational qualification as quickly as I could. I (mostly) enjoyed university and had a full-time professional job in an area I loved before I was 21. There are a lot of advantages to that, but looking back I can see that I was technically skilled but still had a lot to learn, and may have been better with a little less laser focus on qualification and a little more life experience. Pretty sure if I'd somehow been able to accelerate it to full-time job by, say, 18 I would have thought it good for me - but actually it wouldn't have been.