The field of sound reduction is like any other field of engineering. Once you know a bit you quickly realise how vast and deep these things go.
Modern MC's have managed to achieve a high level of sound reduction through a number of improvements. Sound cancelling exhaust, liquid cooling and sound cancelling intake to name a few.
Once you quiet the exhaust, the intake, the gears, wrap the pipe, you still have the air cooled cylinder ringing like mad. There are silicon inserts for between the fins that help a lot. That's about it. Much quieter yes, silent never.
One of these days I would like to really work on quieting mine down but haven't done much yet. In the thread, My bike almost dialled in, I made a easy mod to the stock muffler I am happy with.