Why only one speaker in the rear?.

On a fastback,that opening is actually for a rear window defroster.
I cant imagine two speakers back there firing at each other would give any decent sound.Plus with the rear seat up, um, no I just can't imagine it.
My factory radio just fills the hole in the dash. It's not connected to anything.Never has been.Never will be.
Muscle cars should have 8-Tracks, shouldn't they? Oh how I remember my first 8-track unit. It was summer of 71. The Doors was playing. Over and over and over and........