The AI already has immobile defenses on planets, the guard stations and the like.
Or if you prefer, as far as the AI is concerned, the war is over, the humans are either wiped out or no longer a threat, and it has other MYSTERIOUS PLANNY PLANS it is focusing on. It responds to your threats by tossing a handful of ships in your direction (waves), or to random planets where it thinks you might be (reinforcements).