If the player chooses, he can simply wait for the enemy to come to him, trusting in the power of defense to wear his opponent down. But he cannot win unless he finds the enemy base and destroys it. In other words, StarCraft models total war, or war in which a combatant uses all available resources to the very bitter end. In total war, though, there is no second place, so a strictly defensive stance is a recipe for defeat.
StarCraft is fun; it's just not as politically compelling as it could be. The problem with the StarCraft model of who gets what, when, and how is that there is really only one core value under dispute: the opponent's destruction.2 Rarely is it more valuable to a player to leave his opponent alive and well, but compliant, than to destroy him.
In other words, there are few political options when dealing with external opponents. On the other side of the same coin, a player's control over "his" units is never in question: he can collect and allocate resources as he sees fit, without ever worrying about being thrown out of power for managin his resources unwisely. In other words, there are no internal political opponents to deal with, opponents that could add a fascinating level of strategy to the game.