But surely don't you think there are other planes here now?
Sure there are! For instance, there is the plane where the attacker is thinking about maybe attacking this way instead of that way this time, and the person with whom you are practicing is thinking about handling your attack from another plane!
A good, sincere attack often brings attacker and defender to the same point. All of those planes intersect when the kiai shakes you and the bokken whistles toward the forehead.