> One implies the other. You turn echo off. Then you write asterisks.

That's not how it works. Sudo turns off echo but otherwise keeps the terminal in it's normal cooked canonocal mode, meaning sudo only sees what you've entered after you hit enter. To print asteriks as you type requires putting the terminal in raw mode, which has the addition consequence of needing to implement shit like backspace yourself. Still a UX win worth doing, but it's pretty clear that skipping that and just disabling echo is an easier lazier implementation.