Having an AI that plays like a human is less fun. 'Cheating' is simply how developers can easily balance things.

EX: Often there are units capable of infinite harassment with perfect micro. That's so far from being fun it's never part of actual game AI.

Large base, small waves. It's a simple formula that makes people feel great!

PS: Of course it goes deeper, units need to have basic AI because people can't micro manage large numbers.

I know, but there are even competitions where people write RTS AIs that can do micro like that, and humans still often beat them. Some game developers have great lengths to make good RTS AI that plays fair and like a human, and they are still easy to beat for skilled players.