> Go ask Neil Gaiman or Stephen King if they know markdown and after explaining what it is ask if they need it? And I'd be willing to put money on them just stating "why not just make my italic text italic, why do I really need to put special characters around it?"
Gaiman has at least in the past stated he prefers to write his first drafts with a fountain pen in a physical notebook, to the point where there's various lists of the specific brands of pens he uses.
Stephen King prefers a Waterman fountain pen, though he's also been known to use typewriters, and may very well also use word processors. He's known to use Word for some work, certainly.
But because of the tools a lot of novelists use, a lot are used to using special marks to indicate the (very limited) formatting they do. Many people use multiple tools, including pens, text editors, smartphones, typewriters, or whatever is to hand. For some picking tools depending on what they work on is part of the process.
You may be right that most professional writers today use Word, but novelists makes up a very specific subset of professional writers and have very much idiosyncratic ideas about their preferred writing environment, ranging from the aforementioned fountain pens on anything from loose-leaf paper to very specific notebook choices, to old typewriters (some insisting on specific models of manual typewriters), via long outdated word processors, to modern word processors or editors written specifically for novelists (like Scrivener etc.). Or tools like the one linked here.
The point remains that in terms for formatting, Markdown is sufficient and simple. That does not mean it will be what everyone will prefer, like or even tolerate. But it has all the functionality needed to represent the formatting done in a typical novel, with minimal interference in the writing.
