I write on the computer, mostly. Before that, I wrote directly on typewriters. In those days, before about 1985, we would literally cut and paste text, using scissors and glue. Now we can move large or small amounts of words around very quickly. Revision is so easy, on this mechanical level (changing the words in a text and producing a clean copy), that writing should be getting better and better. There is no excuse for not writing much better using word-processor, which allows for much quicker revisions.
Yet writing does not improve. The mechanical ease of revision does little to help the writer who doesn't know how to write in the first place.
Turner and Thomas de-emphasize revision. Their reason, as I interpret it, is to avoid the fallacy that perfect style arises from tinkering with inadequate sentences until they are good enough. Why not write good prose in the first place? Then revision would be editing, tweaking, fine-tuning.
In my case, the answer to this question is "because I am lazy": I somehow think I have to take notes in a very bad style and then laboriously convert them into finished prose. After all, revision is so easy on the word-processor that there seems little point in putting half-way decent sentences down on the screen during the initial stages of a project. This is laziness that creates more work, because I am always having to slog through messy documents.