Emob78: I find AI in RTS games to be pretty limited, which causes a victory seeking player to always do the same thing to win. AI usually either does early base rushing or cheats in order to turtle long enough to get late game uber units. So in many RTS games, the strategy is almost always one thing - base rush before the AI can. Turtling against an AI, especially one on hard difficulty, almost never works. But that kind of kills the fun because you spend a quick 10 minute game overwhelming your opponent with level 1 peasants or grunt infantry.
So I guess the reason why RTS AI has always been limited is because of the nature of the genre. The point of an RTS is to seek out your enemy's base and destroy it. AI can only do so much with that premise. And as some have pointed out, that's what helps make turn based games more flexible.
That's up to speed. Crank down the speed (How fast units move, resource income, build times etc.) and suddenly playing as almost a 4X becomes possible. The biggest reason RTS is a small genre is the developers obsession with them always being fast if you ask me. The faster they are the more important reflexes are and the less civilization building features are possible unless really good automation functions which are extremely rare.