Why does a headphone attenuator need separate resistance for each side instead of a shared resistor for both?
If you do as you say there will be interaction between the left and right channels.

Any signal in one channel will result in a voltage across the resistor that will then excite the other channel.  Stereo systems go to great lengths to minimize crosstalk between channels.  Ideally there should be less than 1% of the signal in the left channel getting into the right channel and right to left (this is about -40dB).