We say Golf, and should continue to say Golf, because it’s a 'convention'.
Precisely! The
Convention on International Civil Aviation, drafted in 1944 by 54 nations lead to the ICAO which established the core principles permitting international transport by air. Because it related to international air transport the ICAO call sign standard includes the nationality character(s). As private flyers not engaging in international air transport it is, indeed, irrelevant most of the time, but the state expects us to conform to ICAO standards in this matter - because of the
convention.