What you're describing is a bubble, or put more technically: gross malinvestment. It's happening for the same reason any bubble happens, artificially cheap credit financing a speculative boom.

Through government financed student loans, we make money available for any sort of educational pursuit; and unlike a loan for, say, opening a new restaurant, no business plan or market research is needed to convince a banker that the venture is a sound one.

So, the reason this is happening in a "supposedly rational, maximally efficient — or close offer — market economy" is contained in the first word of the description: supposedly.

In truth, government loans distort and destroy a market economy. Cheap credit undermines rational planning.