If i had to make a guess, they either:

1: Do exist, but in a area we don't have access to.
Or
2: Only exist because of a specific Event which only took place on the first.

From what i know and remember (correct me if i am wrong), the Pixies settled in on Il Mheg after the fall of the Voerburt Kingdom and turned its remaining citizens that didn't left into those humanoid grass bushes we see all over the place.

Pixies also seem to be born from the souls of dead children, and unless there is some sort of sidequest i missed out that states othetwise, there is no further information on this to prove Theory #2.