Here's an important and fundamental disconnect that novice writers often struggle with, and that more experienced writers often forget about:
We write, not to show that we know stuff, but to communicate an idea. Some may take exception to my particular phrasing, "to communicate an idea," but I'm willing to stand by this phrase. Student writers trip over this disconnect all the time. As students they're used to taking tests and quizzes to show that they know stuff, to prove that they've done work. So, it's not that unreasonable for them to approach writing in the same way. After all, their papers are usually perceived as just one set of percentages in the breakdown of their overall grades. They approach their writing thinking that the goal of the paper is to show the professor that they have knowledge.
As an aspiring academic somewhere between student writer and seasoned professional writer, I have to keep this distinction before me at all times. I have to think consciously about the fact that when I write, I'm not trying to show people that I know a lot about a given text. Who cares? I'm communicating something of significance about the text to my audience.