That is a good question, and the explanation is fairly simple: The engine is governed. It does not change speed or exhaust note when you adjust the mixture. So you won't hear anything.
It is literally as if you are blindfolded in the middle of a room, and you only know you're at the wall when you bump into it, but as long as all the walls are outside of arms length, there is no way of telling where you are: Left, right, centre, it's all the same untill you reach a limit and then the engine stalls.
Measuring the exhaust gas "quality" for the first time ever allowed me to make targeted adjustments.
The only thing I knew were "lean=> hard sudden stops" and "rich=>stops that more or less announce themselves" but inbetween, as long as the engine was running, no way of telling where I was. Because of the "lean hard stops", the natural tendency is to err on the safe side and that's how I ended up with a rich running engine.

Of course, there are tell-tales like the colour of the oil residue, or the pollution level of the spark plug, but those indicators literally take HOURS of runtime to reveal themselves, and are at best suitable for confirmation, not as a basis to make adjustments.