Why do fiction writers obsess over picking just the right word or phrase?

That’s really all I’m talking about; in most cases there are two reasons to go into great detail on the descriptions - it tells something useful about the characters or setting that’s useful for understanding something now or later, or as a sort of method of slowing the pacing of a book.

Some writers are stuck in that mental movie and are describing it as if it was a movie; I have a suspicion that they’re not mental movie readers, but are writing for that sort of reader. If they were mental movie readers, they’d realize that anything left out gets sort of filled in or glossed over without needing to be explicitly described.