Why doesn't ChromeDriver require Chrome or Chromium?

Of course ChromeDriver requires Chrome or Chromium.

As per ChromeDriver - WebDriver for Chrome ChromeDriver is a separate executable that WebDriver uses to control Chrome.

Now, as per Requirements - ChromeDriver the server expects you to have Chrome installed in the default location for each system as follows:

_{¹For Linux systems, the ChromeDriver expects /usr/bin/google-chrome to be a symlink to the actual Chrome binary.}

You can also force ChromeDriver to use a custom location by setting a special capability. You can find the documentation in Using a Chrome executable in a non-standard location