I want to adjust my app volume and be done with it, I dont want to adjust my master volume everytime. Besides if I turn down my master volume, now my music is quieter, I have to adjust that, people I am talking to are quieter and I have to adjust that.
In my setup I have a volume knob for each of these on a macropad, but I just turn down the desktop audio knob when starting a new game up, then reset the volume back after I adjust the in-game setting. So I personally have worked around it, but 99.9% of people wont have this so they have to use the OS volume control, which makes this a bigger annoyance.
If apps just didnt start at 100% it wouldnt be an issue, too quiet for some or too loud for some is better than max volume for everyone, which is guaranteed to ear rape some people.