It's simply because it was an obvious bug in the AI code. A feature that is only accessible through temporarily enabling auto-battle is a flaw in the mechanics, not a feature. The fact that the advantage of 2-hex units is offset by tactics was deliberately implemented by the game developers. Their logic is understandable – the tactics skill allows you to position troops within N cells before the battle, and it's not clear why larger creatures should get more space to maneuver (+1 cell) right in front of the enemy. But, giving a similar option through the interface wouldn't be so bad. It might not be very intuitive, but with enough effort (in my opinion), it could be done.