They are still learning. So at some point their AI should explore worker rushes, but also explore how to defend against them.

And once the defense gets good enough, the attack portion of the AI should probably do the worker rushes much less often.

Of course, given that the AI has different constraints in micro-management and attention, and that the game is balanced given human skills, some things that would be totally bonkers to do as a human might work for the AI.