Confluence's editor is truly awful. I've spent 15 minutes just trying to remove superfluous newlines -- it's maddening. If you do something like {{fsck}} to create a bit of monospace text, it's totally unclear if there is a space after the text, or there isn't, due to the way the reverse video section is larger than the actual text. God help you if you accidentally backspace into the monospace section, everything you type after that is monospace. It's extremely frustrating.
I wouldn't be surprised to learn that many folks when faced with the prospect of updating a document using the Confluence editor will instead just find something less terrible to do.
I often write my entire document first using the old style Confluence markdown, and when done I import it into a new Confluence page. You can't take an existing document and convert into markdown, but if you are the only person working on the document this is a workable approach.