Isn't that because the only static element of a dementor's appearance is its black, concealing cloak, and that overlaps neatly with the Grim Reaper portrayal of death?
You say that like Rowling had no choice but to use this well-known image for Dementors. Also, they're supposed to look somewhat like corpses underneath.
New chapter!
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 102.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: