The problem with that is, most counters to first tier units rely on splash damage. As zerg for instance, if your opponent is rushing with lings or marines, you

The problem with that is, most counters to first tier units rely on splash damage. As zerg for instance, if your opponent is rushing with lings or marines, you need banelings. But it would be trivial for an ai to neutralize the splash damage, so that wouldn't work. Generally speaking, the counters are balanced for human players. Biased toward high level players, but still human.