I think you've misunderstood why someone would prefer Scrivener or a word processor: a rich-text ecosystem that "just works", in terms of WYSIWG / rich text editing; being the de facto standard for many non-technical user; and being more than "as simple as Markdown", but actively providing a tight ecosystem focused on writing.
Is it possible to recreate some of this functionality in emacs? Definitely. But, it requires a lot of patience, exploration, and determination.
Your editor wants to add track changes and inline comments to your manuscript draft. How do they do that in your org-mode file?
You've received a .docx draft from a friend, or copy for your book blurb, or a press release for your upcoming publication. How do you edit that .docx and return it in better shape than you found it?
You're writing a novel, and it has extensive research notes, background material, and miscellany you're keeping track of. How do you quickly navigate between these multiple sources of content, marking things up, merging different aspects of documents?
I'm a die-hard emacser, and I do a lot of personal and academic writing in LaTeX. But honestly, have you ever had to reformat a LaTeX manuscript for journal publication? It can take hours just to get the damn file to compile.
emacs is worth the time, if you're inclined to tinker and invest the effort to get it to work—but for me, at least, that's a hobby. It's disingenuous to present it as a real competitor to industry-standard workhorse word processors. And most writers with a day job can barely find time to write, "investing time in emacs" is a nonstarter for getting things done! In fact, I tune my own org-basaed PIM system as a way to avoid getting real work done...