Also, the dimensionality of the state space in a "continuous" game is orders of magnitude larger. In a game like chess or Go, you may have dozens or hundreds of moves available at each turn, but only a few of them will be "locally optimal". In StarCraft, there are many more degrees of freedom -- attack timing, positioning, formation, banking versus spending resources, and so on. A good AI will need to be able to abstract that huge state space down to something more tractable.