In Scott Alexander's case, it's because he's into ideas (e.g., eugenics) that most people find reprehensible, so he has to couch them in a lot of bullshit to make them more palatable, and to give himself some credibility.
IME they're usually the first thing you see when you get to the comments, so the really long essays are possibly a way to bore people into not getting to the comment section.
IME they're usually the first thing you see when you get to the comments, so the really long essays are possibly a way to bore people into not getting to the comment section.