Prior to the simplification the laws used to have “The middle line of a scrum must not be within 5 metres of the goal line. A scrum cannot take place within 5 metres of a touchline“ (which was itself added in 2002). So even then it wasn’t explicitly stated that the mark is on the 5m line but was generally just accepted practice from what I recall.
To be fair, back in my long-gone playing days I never thought of this - the scrum was called and we set up where the ref/PE teacher pointed. And you never questioned it, not because of the ‘sole authority’ but more because if you annoyed him the hot water to the showers got turned off.