When games are released, thay have been in developement for a long time as a rule. During that time, hardware changes, but the games are written and Optimized to the existing Hardware. Should it need further optimizing for new hardware, it will be done in the form of a patch. Point is, they are generally optimized to play across as wide of a platform as they can. With all of the possible PC configurations, you can imagine the job of trying to hit a happy medium to cover as many as possible and keep the quality up.