I don't want to adjust my system volume to change the volume of an over-loud video. My system volume is set for my comfort so my notifications and other applications are the right volume.
A volume slider on a video player is a really basic feature that every one should have.
Set for your comfort doing what exactly? You're basically just prioritizing one source of media over another
As TeMPOraL has pointed out you only listen to one thing at a time. Notifications etc. also have their own separate volume settings generally.
And you only ever listen to one thing at a time, from one program at a time, and all audio content ever has its loudness perfectly normalized to a global standard everyone adheres to.