You're right in that with a physical control you still probably glance at it as you're reaching for it, but you don't have to keep your eyes on it the whole time unlike a touch screen. With a touch-screen there's no feedback that your finger is on the button and that you have actually pressed it. With a physical control, button, knob, or slider once you have your hand on it you can manipulate it without looking. They demand momentary glances, not seconds of constant focus.