I suppose it's because Grand Slams are the events that draw all of the top players, which makes them the most competitive tournaments. I do think that the five-set match is a fair determination of who is better in a head to head match, and certainly, a player's level of physical conditioning is an important part of how good a player they are. If you take the player's physical conditioning out of the equation then what do they play? Best of three sets? How about just a single set and the winner moves on? Physical conditioning is just one part of the equation along with all of the other things that make a player great; speed, agility, mental toughness, shot selection, strategy, etc.