Because it's not just the performing artists that would need to give copyright permission, it would also be all of the collaborating artists (lyricist, etc), and the label, and potentially the mastering house. It would be a licensing nightmare to get your song on the air - and would probably be harkened to the bad old days of payola.
Read up on the history of copyright law, especially as it pertains to music.
But all of these entities would seem to have the common interest of maximizing their revenues from royalties. So why shouldn't they have the right to authorize one party (perhaps the label) to negotiate on their behalf?
Also, it's becoming increasingly easy for musicians to digitally produce their own MP3s, so if there's no label or studio involved and the musicians wrote their own lyrics, it would seem to be even easier for them to negotiate on their own behalf.
Read up on the history of copyright law, especially as it pertains to music.