It's not about complexity in the sense we're familiar with. Minecraft isn't more complex than Starcraft from a human intelligence POV. Kids can play Minecraft. It's about the difficulty of fitting it into current methods. We can solve Go because we have a symbolic-neuro planning approach (monte carlo tree search over an evaluation net) that almost perfectly models the correct way to reason about the game at an expert level. This is an incredibly strong inductive bias that gives AlphaGo an unfair head start over AlphaStar. Starcraft is harder to solve because we don't have such a symbolic approach figured out, and we need the net to learn visual representations and connect those visual representations to actions, and we have a continuous action space. So good luck with that! Minecraft is even harder to model because the rewards are so sparse.