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.

https://rationalwiki.org/wiki/Scott_Alexander

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.