I would guess a fairly big factor is because generally the game logic which runs of the processor doesn't degrade well. With graphics you can lower resolutions, change texture sizes and add additional lighting effects which are optional so the game just looks a bit worse but plays the same. Trying to do the same with game logic is much harder, maybe some adaptive AI could be made to play better on faster hardware plus some extra graphical effects probably need some extra processor time but these changes would be much less.
Then the developers want the game to be playable on as many machines as is feasible so they head for as low a target as possible which is a single threaded machine. Making it work on multiple cores then turns into a pretty difficult task for very little gain because of the above.