Why is academic writing so bad?

According to the stereotype, academic writing is at turns dry, jargony, esoteric, discursive, self-conscious, inward-looking, and—worst of all—just plain incomprehensible. The purpose of writing is to communicate ideas clearly and concisely, but academic writing achieves the opposite. In short, academic writing is bad. Every researcher knows there is some truth to this stereotype but also plenty of exceptions. So why is academic writing often so bad, and what distinguishes the good writing?

This is a companion discussion topic for the original entry at https://www.epatters.org/post/academic-writing