I think it's the difference between the cinema and watching a DVD on your own at home. Yeah, you can get engrossed in the story at either venue, but only the Cinema runs the risk of some dolt kicking the back of your seat over and over or a 7 foot person sitting in front of you and through no fault of their own wasting an hour and a half of your life.But the question really should be, why not BOTH?
One day someone will design a high level "hyrbid" game that interweaves single player and multiplayer into one full gameplay ... and that game will blow people away.
Imagine a game that requires both to finish ... a game where other peoples choices will effect the world you are exploring, even as it unfolds for you. A truly unique gaming experience every play through thanks to dynamic changes in the world and story by others.
Be disappointing no more, for my cars mileage is now: