There is no such thing as a perfect score either.
A 10 score doesn't mean the game is perfect, it just means it's better than all of the other games it's competing against. Games aren't scored on how close they come to perfection.
How would you determine what "perfect graphics" were? You would have to if a 10 in graphics meant they were perfect. What about "perfect sound"? Would that mean they were clear, well recorded, the best artistic match for the graphics, or simply had the most accurate electric guitar sound?
You are right, there is no such thing as a perfect game, but then again, there is no such thing as a perfect score either.