Oh boy... hair. Once more into the breach!
A) Consistency. Due to asset limitations (licenseing, availability etc) in TRS1 a lot of people didn't have hair thus it carries on to TRS2. dForce hair editor wasn't a thing when I started. And fuck dForce strand hair, it's resource hog.
B) Hair mesh breaking. Outside handful of hair assets most breaks when used on non-human head shapes. You can try to fix it to a point in post, but when you produce 1000's of renders (14k for Sorcerer give or take) it just becomes unfeasible to fix those errors constantly and consistently.
And I do like my aliens having alien features instead of just a different colored paint scheme. Sadly I'm limited to humanoid body but can't have everything. And you only get away using mohawks for so many times... I do have at least one alien coming up that has hair. She has some alien features, but a caveat, she is still at concept stage until she appears in the VN.
C) And in case of Chiel, I just love her looks without hair.
D) Aaaaaand this is more generally speaking, but in a lot of scifi the aliens are hairless unless they are humans with or without new body paint scheme.
EDIT: And cause I can't be arsed to make a new post...
Neesush (The green short stack) has hair. Also Sage isn't a human... she just looks short and stocky one.