Although it is easy to bread roaches, I'm sure Kyouhykou ain't big on Entoma eating them and people running away or being disgusted by them. I'm sure a real brain eater is just an unsightly thing which one of the 41 was as well as one in the dungeon. Meanwhile Rubedo or Nigredo (forget which one) is basically locked in on the frozen prison floor or something simply cause she doesn't have skin on her face. Just muscle, eyes, teeth and a bit of bone