SC2 has a lot of things that help the player over an AI, like imperfect information, a very large ruleset, and continuous movement and time.
The main advantage AI can exploit is "infinite micro" units. There are certain units in SC2 that are primarily limited by someone's ability to pay attention to them. Terran Reapers, for instance, in early patches could dance back and forth just within range Zerg Roaches, but out of range of being counterattacked (though the reapers would quickly die if player stopped paying attention). This required such fast movement that it was almost never employed except by professionals during the early game when not much was going on.
If an AI is not limited by the number of actions a person can take, these "infinite micro" units become extremely powerful. In particular, most of these types of units are within the Terran race and most powerful in the early game. The best AI in the near future will likely focus on early all-in attacks using these sorts of units (primarily reapers, widow mines, hellions, and medivacs). The human player would have to rely on defensive structures that would put them at a major disadvantage and then catch up in the mid-late game, relying on the fact that the AI gets worse as the game goes on and becomes less predictable.
Another thing to note is that Terran strategy relies quite heavily on killing the enemy workers in the early game, but the inbuilt controls for targeting workers are quite clunky and prioritise military targets. Players can target individual workers manually, but generally do so with their whole army at once, while an AI could select the perfect number of units required to 1-shot a worker, give them the command, then select another group, and so on.