A keyboard is not strictly required either. You can use an on screen keyboard with a mouse, text to speech, etc. And on anything that doesn't require text input, it's easy to avoid the keyboard.

The key is flexiblity. On Linux, I could avoid my mouse or my keyboard or my entire graphical desktop if I wanted to and still be productive. Macs don't have that flexibility.

I single out a mouse because it is a very low information density device. It doesn't really do much, so it is the low hanging fruit. Whether a mouse is required by an OS is like a litmus test of "can the user configure the OS the way they want, or does the OS dictate how the user will use it"