>>36648460I think it's just easier to imagine a humanoid creature as a final stage for designers. Maybe it just works better since you are supposed to use them to fight and it gives more opportunities for moves and different animations.
Im just talking out my ass but, pokémon isn't the only franchise that does that, just look at Spectrobes or Inviziamals.
Sadly, neither of them is here to tell us more about it. 