The AI is very... very... stupid. How stupid they are usually is random in a custom game/skirmish, and usually intentionally stupider on your side during Hard/Nightmare tournament. I've had matches where we completely dominate to matches where my AI in a 2v2 singleplayer skirmish runs out and gets killed in the first wave of creeps.

My hypothesis is that the AI leaves the job to you. Since it they have five AIs, I'm guessing the programmers coded the AI to react to each other in coordinated ways. Since you weren't coded by the programmers (I hope), you don't know how to react to the AI the way the programmers want you to and the jobs left by the AI for you are unattended to, simply because you don't know that they were left for you. The AI seems to have a strong sense of coordination when they fill up a whole team, but that really doesn't help the fact that they love to run up and paint a huge target on their forehead.