Undergrads who care about learning and research will take the most challenging classes, do research with professors, and surround themselves with other strong students who will push them.
Even at top universities, very very few freshmen are capable of doing high-quality research immediately. They'd be better served learning the foundations inside and out with a cohort of similarly strong students to challenge them.
To agree with you: I've worked with several really brilliant undergrads doing and publishing great research. But all of them were rightfully undergrads. Even if they were actually capable of doing great research, they benefited from the breadth.
If you have bright enough undergrads, you change the curriculum for them within their field of expertise, so that they still get the breadth of things outside it while not wasting time with things they know. You let them not take as many classes, take graduate courses, do more research, take more courses from other departments in related areas but with different perspectives, and so on.
When I was an undergrad, in physics, there was a professor in the department who had done his undergrad there and was legendary, as was quietly mentioned in awe, for not taking any undergraduate physics courses while there; the department had let him skip all of them, and instead take graduate courses and do research.
I'm not sure that's a simple argument and can't imagine many would agree.
Undergrads who do research generally aren't very good at research yet. A major reason is they either lack or don't fully understand the pre-reqs, which they progressively and cumulatively learn during undergrad. A student can be incredibly smart, but acquiring a strong rigorous math background will still take years.
About pre-reqs: third and fourth year PureMath classes at UofWaterloo consisted of math I already took in HighSchool in Romania: group theory, ring theory. Plus some calculus I already read in high school out of curiosity: measure theory and the Lebesgue integral. Another Romanian guy at UofW was auditing 4th year classes while in his first year (he is now a math professor at an American university)
I can see a committed and gifted student being able to get most of the pre-reqs for doctoral studies in America or Canada while in high school.
If you don't know the foundations well, you don't belong in a postgrad program. That's the reality and how it currently works. Undergrad teaches you those foundations.
Anyone can try doing research, even undergrads who half-know the foundations. However, trying research doesn't mean you have the background to do great research or to succeed in a postgrad program.
Even at top universities, very very few freshmen are capable of doing high-quality research immediately. They'd be better served learning the foundations inside and out with a cohort of similarly strong students to challenge them.