I’ve read plenty of books that could have told the story in 20% fewer pages, or had a swapped character name, or…

Some of these passed through multiple layers of editing. Some of these "passed" through those layers because the writer got too famous or cantankerous or…

I acted as one of my wife’s editors for her books, and some of the things that I checked (as it was recent historical fiction) included whether the weather she described for a chapter matched the weather reported by Environment Canada. She had one scene in early April of a year where she said that the "last of the snows had gone…", but there was a snowstorm on the 3rd of April or something like that. As the weather was not critical to the scene, I insisted that it either be dropped or changed (she changed it) — because getting that sort of item wrong can throw a reader out of the flow of the book.

In another instance in her second book, she had a weather condition that was crucial to the plot. We looked to see that it would be plausible (e.g., it was not 50℃ in February in Toronto in 1979), then let the weather condition stand because it mattered.