Well, some of them. Others use Obsidian, like P.D. Workman, who has written 43 novels in Markdown, many of them with Obsidian. [0] And a few still use WordStar and WordPerfect.
Word is indeed the lingua franca of publishing. To go to an agent or publisher, a doc or docx file is where a manuscript usually has to end up. But "end up" is important here...95% of the work of actually writing need not happen in Word, and many successful writers don't use it for primary drafting.
The professionals use Microsoft Word.