Being an author and professional line editor, I meet a lot of people who've written and self-published their books. And yes, majority of them are really not done well.

And that does not mean that they don't write well. They are probably doing their job as a writer. But the writer will always view their writing from their lens. An editor is a fresh pair of eyes. And proof reading is a precise technique-filled task as well.

Without these, a writer is just that. A writer.