Morbius has been using the reality warping aliens machines for so long that he's shaped the planet and the Krell civilization to his own expectations, dreams and imagination to the point where nothing can be conclusively known either about the Krell or the planet. The world has become Morbius' own Lotus-Eater Machine fulfilling Morbius' desire to endlessly explore a fascinating alien civilization. Case in point, Morbius believes that the Krell brought the tiger and the deer to the planet from Earth's prehistoric times which raises more questions than answers, and yet the tiger both lives in harmony with the deer and later acts in accordance with Morbius' jealousy foreshadowing the Id monster's motivation, cluing us in that tiger is essentially another manifested ID monster.
While Caliban is more monstrous and Ariel prettier, Caliban is a physical creature and Ariel is a fae who commands great magic. Robby himself, while an impressive creation, is ultimately a robot and his powers are limited. The Monster from the Id is vastly more powerful, and while it's grotesque it's bound completely to Morbius, obeying his innermost desires. But because Robby is friendly and the Monster aggressive and terrifying, the audience switches the roles around. More proof is that the spaceship crew can interact and socialize with Robby, while the Monster is invisible (for most of the film) and only the effects are seen, much like Ariel in the original play.
