I really don't think it's that hard to add. Other games have it. i HATE sticking my earbuds under my headset. i would play on the xbox but all my friends are on ps3.
I've always hated it aswell, I would love to listen to my music while mowing down zombies.
There is in-game music, but there is no option for you to beam your own music through the game.
If you are playing online, certainly taking in the sounds and audio of a match is more important than.. music?
I've got the turtle beach xp500 headset, and they have dual-bluetooth channel which lets you listen to game audio and stream music in the background, etc.
If people are really into music, that's a great option to have.