A stereo sample contains spatial information about the sound source.. A human perception of the mix is often thought to be a sound stage where a "band" (sound sources) is playing, each source having its own spatial location and distance and the stage having its parameters (outside, inside - reverbs etc). If you just mix random stereo samples with each other you're going to end up with a completely random sound stage .. same as if you'd do a really bad mix. the human ear can pick up really small clues and come up with a conclusion that the mix is "impossible" and doesn't sound so niice.
Even if it's experimental electronica, the ear is used to certain rules.