A 'perfectly' micro-ing AI is both boring to implement and boring to play against. Somebody made one for SC2 back when that was popular, take a look at these videos to see how uninteresting it would become after the first attempt.

https://www.youtube.com/watch?v=0EYH-csTttw

https://www.youtube.com/watch?v=IKVFZ28ybQs

https://www.youtube.com/watch?v=DXUOWXidcY0

But even the best AIs in Starcraft are still quite bad compared to competitive players.

yeah but I'm pretty sure these bots still get crushed by humans because of macro, planning and positioning, which is why it takes top firms on the planet so long to solve the problem

If the AI wasn't very smart, then you'd expect there to be some optimal path that would beat them. But Starcraft has always been a game with very little skill ceiling on micro. Well micro'd units have the potential overcome huge macro advantages. A famous real-world example would be Boxer's Immortal Marines:

https://www.youtube.com/watch?v=WJp0t9n8DWk

My understanding is that a micromanagement-based AI is likely not to be an "interesting" difficult AI to beat. The DeepMind project is aimed more towards (with its apm caps) at building something that makes good decisions (e.g., unit composition, build orders, crisis management) instead of relying on brute force optimal control of units. While it is likely possible to completely break the balance of the game with perfect unit control, playing against such an AI (which is really just a bot at this point) would get stale pretty quickly.

Just like playing against MarineKing would get stale quickly for most players :P

The problem with starcraft is that its like chess - for a lot of players it really DOES get stale pretty quickly, with only one or two interesting moments in a game (assuming no early blunders). The games that are interesting involve near perfect play for 10-20 minutes and then frequently one split second decision. Macro changes or new strategies are quickly picked up and interated on, and I think thats what the hardcore still stick around for

The difference is that an interesting AI could likely be scaled back meaningfully for lower skill levels. Make MarineKing play with only the mouse against a lower skilled opponent? The results might be less stale.

Rate limiting a brute-force AI would make it trivially bad.

Not really. Stopping MarineKing from sending tons of APM is only part of the challenge. His choices for what to do in any given frame (yes, frame) are also just better than basically every player on the ladder. If you remove his keyboard, you'll find that he still wins most games against folks who don't know the game.