I think the reason for that is that for most music you connect to more than one musician. Then the levels add up and if they are at the same level as the original input the output might get overdriven with more than one partner playing at the same time.
It might be nice to be able to store a volume setting in a group for all members of that group, but the levels might change the next time mics are placed (differently) and not leveled in by the same amount as before.