Because, despite the chasm between what students learn and what workers do, academic success is a strong signal of worker productivity. Suppose your law firm wants a summer associate. A law student with a doctorate in philosophy from Stanford applies. What do you infer? The applicant is probably brilliant, diligent, and willing to tolerate serious boredom. If you’re looking for that kind of worker—and what employer isn’t?—you’ll make an offer, knowing full well that nothing the philosopher learned at Stanford will be relevant to this job. The labor market doesn’t pay you for the useless subjects you master; it pays you for the preexisting traits you signal by mastering them.
I read years ago that the Army could enlist someone who did not graduate from High School due to grade (which likely hardly allowed to happen now) as long as they went the 12 years.