so why put it on the other side of the keyboard? Why not just leave it on the F12 or whatever key?
On the laptop keyboards, where they began this icon/action association, they began at the left and moved to the right. I don't believe it matters entirely where. They couldn't jumble the media controls in with brightness, but now that there are 7 media keys they couldn't leave them on the right above the numeric either as there wasn't enough room. Now all controls are next to each other, playing controls, sound, and eject - perfect for DVD player.
That actual consistency is leaving it in the upper right of the keyboard.
For what it's worth, the key is still in the upper right hand corner of the main keyboard, not off in the extended keyboard world. It also lends itself better to media controls than being off on its own at the end of a long line of F-keys.