I was about to post the same thing as William, but I'd like to expand on it a little bit. It's very hard to write optimal code for the future. Given the choice between writing something that will scale to hardware you don't have vs. writing something that will work on hardware you do have, most people will chose to do the latter. Since the single-core paradigm has been with us for so long, most code that has been written (especially for games where there is extreme pressure to get it out the door) isn't that future proof.
x86 has been very kind to game programmers, since we haven't had to think about the ramifications of less forgiving hardware platforms.
