I think it's because of the general nature of the prefixes of the words. Co-pilot says that the two of you are doing it together. Co-operatively. Auto = auto

I think it's because of the general nature of the prefixes of the words. Co-pilot says that the two of you are doing it together. Co-operatively. Auto = automatic = you no longer have to do it. Think of an automatic vs. stick shift car. And automatic car you no longer have to shift.