I don't know where I learnt it but some time back I was taught that the human was only really capable of hearing two things at once. In the case of the piano trio we really only hear the piano and bass and feel the drums. As we add more voices to the arrangement more of the original parts move from being heard to being felt. Starting off with 5-6-7-8 different things, all at once and all of equal importance is going to lead to tonal confusion. Just hitting the audience with a packed, canned backup style and the melody is never going to have the effect of a carefully arranged piece in all of its parts.