You answered your own question here. Most “high performing” kids will lose interest in one of the foundational branches of math and will decide instead to read fantasy or build a potato cannon.
Without the structure stressing the stuff that is truly important despite it not seeming like it at the time, the vast majority will land in college needing to take remedial high school courses in math and science.
I know that my own children don't master subjects that way and it also doesn't mirror adult work.
As to the why things are the way they are, people don't wanna do better because it's inconvenient.
In the current US system, it's hard to justify focusing additional resources on students that are already performing well.
As another commenter noted, it's the privileged kids who get opportunities like that. Which is a real shame because it's an environment that lifts those who need more flexibility and personal support.
Schools that are up to date on this and follow advice exist, but are expensive and rare - which also attract parents who do not need to sit at a desk or stand in front of a CNC machine at 8:30am. Gordonstoun/Schloss Salem comes to mind.
College wise, what I recall was a lot of freshmen who just couldn't be assed to wake up for class or do homework without parents on their case. Especially with the dorm basically being a 24/7 LAN party, and I recall one dude who just watched like, every DVD ever. The coursework itself in freshman year was comparatively breezey.
For school before then, I suppose it depends on your definition of boring and independence. IEPs for gifted kids can put you into advanced classes of all kinds. It's still very much on rails, so I'd liken the independence to a choose your own adventure book. And, sure you can do AP English instead of normal english but you have to read like, Bronte and Hurston, so the boring is only far off.
I don't have anything to do with it (yet?) but our sister institution is Alpha School - https://alpha.school
They're trying to change schooling to be more efficient, engaging, and self directed.
My understanding is they're far from alone, and there's a lot of interest in overhauling traditional schooling at the moment.
I mean, there's been interest in overhauling traditional schooling since forever. However, a significant up-tick in (temporary?) home-schooling during the pandemic really got parents thinking about education methodologies.
The science around teenagers needing more sleep, and doing better in life/school/everything if they start their day later, seems well-established at this point.
Additionally, there's plenty of known-better ways to learn beyond the rote-memorization-and-standardized-test system that's still de rigueur.
Are there no schools out there that put these things together and make an attempt at something better, especially for high-performing kids? What if school started at (say) 10-ish, and it gradually became more and more self-directed as kids headed towards graduation? What if it also mixed in all the latest cutting-edge research about how kids learn math/science/languages/etc? What if it also got creative with the curriculum and mixed in all the topics that people always say should be in schools, like "how to do your taxes" and whatnot?
I remember getting to college and being wildly unprepared for everything. School before then was devastatingly boring and fostered very little independence. Then college came along, and it was both brutally difficult and had almost no guardrails of any kind.
It was quite a transition, and it makes me wonder why there aren't more varieties/experiments around radically different approaches to schooling. At minimum, for high-performing kids, you'd think the modern world would have something.