IMHO, part of that is that you don't want more threads than processor cores (unless they're IO bound). If we had lots lots more cores, then I would expect to see physics, AI, game logic etc use more than one thread. There is little reason to pay for the additional complexity, when most mainstream computers don't have more than maybe 4-8 cores. Obviously not everything parallelises well, so the full extent that this is possible remains to be seen.