You're assuming way too much about game development with this statement.
If the Ice Climbers require two completely separate models to be running simultaneously in reaction to one another, with one human-controlled and one AI-controlled, then that takes up a larger chunk of the system's processing power and RAM than, say, a character model made up of three pieces that don't act independently of one another. At that point, it's just one character represented by "three" on screen. Like a Dugtrio in Pokemon Stadium.
Not saying Chorus Kids are in. (I really don't want them.) But blanket statements like this really need to stay out of debates or everyone's going to rush in to try and refute them, thus spawning more arguments and less actually meaningful discussion.