Right, you can't build the AI this way and expect to succeed. In Chess, there are a maximum of 32 pieces sharing 64 possible locations. In Starcraft, there could be hundreds of units on the map at any of millions of possible locations. Chess is a complete information game, Starcraft is not. (Your knowledge of your opponent's position and strategy is dependent on what you can see and largely on indirect intuition about what you DON'T see.) In Chess, turns are taken and there is plenty of time to compute each action optimally. In Starcraft, there are orders of magnitude more possible actions you could command, and you must choose which units are most important to focus on at any moment.
There's just a fundamentally different approach required. And I think a much more challenging one.