Our sources are all monophonic. Single point sources of sound without any directional cues whatsoever. A violin, voice, horn, or any acoustic instrument I can imagine is mono, yet our systems require two channels to properly reproduce that monophonic source.
The difference, of course, is positional. Where in the soundscape does that monophonic instrument reside? Our 2-channel ears, like our 2-channel eyes, capture the monophonic source from slightly different angles and distances, adding perspective to the mix.
Because each ear is judging what it assumes is a single mono source, it is essential that reproduced sound between channels be as identical and independent as possible. The left channel reproduced mono needs to be independent of the right channel’s presentation, and both have to be as true to the original as technically possible.
Deviations from sameness, as well as interactions between the two channels, are injurious to a proper spatial illusion—a good argument in favor of mono amplifiers and excellent channel separation.
Attempting to reproduce single-source mono with 2-channels might seem counter-intuitive, but for the moment it’s all we’ve got.