We want one class of persons to have a liberal education, and we want another class of persons, a very much larger class of necessity in every society, to forgo the privilege of a liberal education and fit themselves to perform specific difficult manual tasks.
K-12 teaching is a low-status profession in this country, and in academia, teaching (what is already known) is less highly valued than research (expanding what is known). Teaching teachers is then the lowest of the low, totem pole-wise.
