"Again, a grossly simplified analogy but, fwiw, you can hear what is behind you but you cannot see without turning your head." - Kr4
In the spirit of taking things too far...
If someone turns a lamp on behind you can generally see the light that comes from it reflecting on things within your visual range. Just because you can't see the source doesn't mean that you can't see the light. In some ways the light from a lamp and the sound from a speaker are similar. Maybe that's why people get confused and use lamp cords on their speakers.
In the spirit of taking things too far...
If someone turns a lamp on behind you can generally see the light that comes from it reflecting on things within your visual range. Just because you can't see the source doesn't mean that you can't see the light. In some ways the light from a lamp and the sound from a speaker are similar. Maybe that's why people get confused and use lamp cords on their speakers.