This would slow down the game a LOT as well as introduce an unnecessary amount of new provinces to the maps. People with multiple accounts could still do the same thing, but slower, and actually a lot more impact.
Image this:
Player A - AI - Player B
both attack AI, then Player B turns around runs into the other AI and player A gets the entire AI + player B's provinces.
AND NO ONE CAN STOP THEM, precisely because they are separated by AIs