As a long time Chess player, I will simply have to disagree with this. The game itself can be mastered, and the ability to consider several moves per second is an extremely relevant portion of the game. Grand masters do not become grand masters simply by knowing their opponents -- in fact, in most cases, you do not know your opponent or his playstyle.
This isn't the case. Chess designs itself deliberately this way; that is, "mastering" chess is specifically the act of becoming a grand master. Diablo does not do this -- it simply isn't how Blizzard designs the game. "Winning" in Chess is speficially the act of beating the other (good) players; "winning" in Diablo is... beating Diablo, and perhaps farming very good gear.
What you are essentially asking is that single player games be held to a different standard than multiplayer ones, because in multiplayer you have to play other people and that's not a fair comparison. I simply reject that notion; what I care about is challenge, and I would be perfectly happy for that challenge to come from a computer (ChessMaster on very hard is certainly that) or from a person. The fact that single player games tend to be static -- with observable patterns that one can eventually recognize and overcome -- is not a justification to be treated differently. It's simply a reason that single player games tend to be easier and why I tend to stick to multiplayer games, because I'm a person who considers challenge to be extremely important to my gaming enjoyment.