The most informative answers I've encountered on StackOverflow are either a product of research (benchmarking, analyzing multiple sources) or very specific knowledge, sometimes written by the author of the framework/library in question. I'm not sure your analogy applies since these answers demand substantially more effort than the (usually) predictable and repetitive questions teachers face in class.
At some point someone has to write it, like school textbooks. After that your just looking at distributing that knowledge. The replacement for search engines solves the latter problem.
