I think it's more because a game is very sensitive to jitter in CPU performance, and can reflect pretty well the responsiveness of the machine with a heavy load in the background. You can replace the game with scrolling a webpage, using slack during compile times, etc. With a game, you get a hard number (FPS) and a very visual indication of how well the load is doing (e.g. stuttering is very noticeable and is a pretty good indicator). So the game is not the point, it's IMHO a visualization of the effectiveness of the scheduler.