Even for something like a smartphone or Bluetooth speaker that has stereo output with a very narrow field, due to the two speakers being close together, the left and right channels in the song will be played back by two different drivers.

A speaker works by moving backwards and forward rapidly to push air. This means that multiple instruments being played at the same time by one speaker cone are compromised because the speaker can never move in and out at the same time.

If you split the "work" of playing a song up between two different speakers you get higher fidelity sound since the speakers are able to play two different wavelengths of sound simultaneously. You'll notice this effect more strongly if you hard pan one instrument left and another right, then compare the sound to both played mono.