Let's take Star Wars for example. What makes a Rodian a Rodian. Is it just the creature's looks and appearance? If a Rodian started flying like a Geonosian from cliff to cliff, everyone would wonder what the heck is happening.
Creatures in stories like BG3 have set characteristics. Remove those, and the creature loses its original identity. Intellect Devourers that no longer devour intellect but act like thugs for Mind Flayers causes them to lose their original identity.
So, again, why use a monster and strip their identity when there are plenty of other monsters you can use that fit better?
