> Once you learn that you can pop into a quick order menu with this specific sequence of five keys, you just automatically open the right menu the moment a customer walks up. No thought, just pure reflex.

This works because you can 'buffer inputs' as the gaming crowd says. You can hit the keys and the computer reads them one at a time and does what you asked at its pace. Often these kinds of systems do run faster than the input, but when they don't, it still works.

It's hard to do that with a GUI, you usually can't click (or tap) the button before it shows up and expect it to work... and when it does work like that, it's often undesirable.