To some extent it depends on the game.

First person shooters can have super accurate AIs that are really frustrating to play against. Try beating the last "boss" in quake 3 on nightmare difficulty. A game like Starcraft relies much more on decision making that raw 3d math calculations and that makes making a decent AI pretty difficult without giving it advantages like extra resources or vision. That said, even though a game like Quake is easier to write an AI for, top players can still be top level AIs with superior reasoning about using the map and timing power up spawns and such.