All they do is read a bunch of books and write essays on them, they almost never take actual grammar / grammar theory / copyediting classes, so why the heck do they think they are some kind of authority?!? Simply having an English professor (who doesn’t necessarly know real grammar rules) correct your grammar mistakes in the essays does not mean that you actually learn grammar.
It’s naive to believe that taking one class in grammar does makes you an expert. Even if it did, most undergrad English programs don’t require it.

Image taken on 2008-05-16 14:06:57 by Tony the Misfit. Image Source. (Used with permission)