It’s one of my favorite pieces of spelling trivia.
People assume it should be spelled “just desserts” because of the pronunciation. It’s spelled “just deserts” because it’s an antiquated noun derived from the same root as “deserve” (and it’s pronounced like that for the same reason).
No, I’m the type of person who knows interesting trivia and backs it up with sources when called out by people who are too lazy to do a basic search:
https://en.wikipedia.org/wiki/Desert_(philosophy)