Because you can have more than one application with sound output. I like listening to music while I play games; I set the music to 80%, and I set the game to 15-20% volume. This cannot be implemented in one single volume control.

It would be nice if all your running apps volumns could be controlled from a single place though, rather than having to hunt around for menus in each app.

There does happen to be at least one operating system that supports this: Windows. ^___^

Yep, since at least Vista, I think. The UI guidelines recommend against in-app volume controls for everything except media players:

http://msdn.microsoft.com/en-us/library/windows/desktop/aa51...

This doesn't work well with games. Yes, you could change the volume settings from there, but most 3D games are full screen and it's not usual to alt-tab the game just to modify the volume settings.

On Linux (specifically Ubuntu, but I believe generally relevant) PulseAudio controls the volume of the loudest sound by default, but you can also get a panel showing each input and output from the volume menu -- two or three clicks, vs. two for Windows.

pavucontrol for pulse audio

Pavucontrol well also let you assign apps per device. Windows has the capability of switching app output devices but sadly only uses it to make them follow the default.

Because computers and software work by linking together components often without sufficient communication between the pieces to easily centralise operation.

And unfortunately a centralised volume control might require defining standards. I wonder if there is a way to hack around that.