ncurses lets you move the cursor to arbitrary positions, and also lets you hide it. And you can disable echo, so you can type without the cursor having any influence on what you see. This means you can treat the terminal cursor as though it's a software-rendered cursor in a GUI app and apply all the same rules. But nobody considered doing this when Emacs was first written (maybe it wasn't possible then), so the assumption that the cursor is always on screen is difficult to change.