Because the end product will have only left and right channels, and 'mixing and mastering' is a huge part of the creative process, not something that can be don

Because the end product will have only left and right channels, and 'mixing and mastering' is a huge part of the creative process, not something that can be done by the 'player'. And of course 'IP' - though some somemakers do release content in this format in the hopes of remixes, many do not want that out there.