Earlier quoted context omitted.
Because people don't just learn to be doctors, physistics, biologists, structural and civic engineers, chemists, and countless of other professions on their own (not in any significant nunber after the 18th century at least), they learn it in university, with a structured curriculum, theoritical tuition, supervision, labs, and so on.
The vast majority of people with degrees will not use that degree in their job. I certainly didn't and looking at my fellow physics or maths grads the same is true of almost all of them.
Because I guarantee that the "vast majority of people with degrees" in the professions I described and many more, will use the knowledge acquired during their degree in their jobs.
I'm not sure what physics or maths grad doesn't use the math he was taught/practiced/and picked up during his tutoring for the degree.
The implication is that they learn those things before going to university? Or that they learn them in grad school? (as if studying for graduate school is not also university, and as if the prior knowledge acquired during their BSc and such wasn't necessary to understand what they encountered in grad school -- and to have a basic understanding of their field).