Only because you seem to refuse any alternative or different visual.
BTW the mob rules in the DMG are no more "optional" than any of the other rules in the book, it's under Running the Game > Combat > Handling Mobs. The word optional appears nowhere in the text. But even if it was, so what? The book gives you ideas how to handle it.
You have a very specific visual that only appears in a tiny fraction of literature and you're complaining because the rules don't support a very specific niche case. When it actually does.