Why the best editors still do multiple reads
You may think that a premium editor can read through a work once - whether it's 500 words or 80,000 - and then you should be able to publish.
There are quite a few reasons why this is probably not true for 99% of works.
1. Authors think give editors work they haven't read through after writing even once.
This might show up in the form of multiple unfinished thoughts, ideas that are repeated in the next paragraph, and "obvious" mistakes. Some of these issues may be able to be fixed by the editor - and others may require input from the author, but either way the author would probably have caught it if they'd done a separate read through from their writing session.
2. Authors are on a self-imposed rush.
It's exciting to hold a complete draft in your hands! A complete draft isn't a finished work, though. If you're making up deadlines that you try to impose on your editor when you don't have any idea about the work you need done to make this a polished product, it's like saying, I want a house built by August 7th and by the way it's July 25th. Even if you think you've made a great deal of changes to your work already and you've actually read through it seven times on your own, there may be issues your eyes are blind to in your own work - which is why you're hiring an editor.
3. Editors are human
Sometimes I get distracted by all of the spelling and grammar issues and I miss a plot hole. Sometimes I get distracted by a plot hole and I forget there's no end quote on some line. Multiple read throughs let me catch things I might miss after 6 hours of editing when I stop to look up a word I'm not 100% sure about the usage of, and come back without seeing a fifth mistake in that sentence. It happens - but you don't want your book to contain any mistakes, if possible. So allowing the proper time for editing your work is important!