Humans usually cannot take books or a printed compilation of all the chess games in history, while computers have a database with a lot of chess games. If humans could take a whole library with them to the match, things would be different, but then, the reading/writing speed of a computer is much higher then the human reading/writing speed, so I think the comparison between human muscles and a gasoline engine is fair. Kasparov playing with no time limit and his whole library at hand would still be a match for any computer.