Because you need skills to be employed. The ridiculousness of the situation is such a plumber is probably spending their time more valuably than say who studied Norse Mythology. Note I'm talking more in economic terms only. Sure the person who studied Norse Mythology knows more than a plumber, but you need a plumber more often than you need some one who knows Norse Mythology.
>>Why can't you say just the same thing about, say, teaching people to read?
That sets the basic platform for other stuff. You build on top of it.
>>Fifth-grade literacy is probably enough to do many jobs, so maybe we should toss out high school too.
If I was spending $X00,000 on a degree and expect a job in return which some one with fifth-grade literacy can do. I'd be seriously worried about my choices and decision making skills in life.