@Qelix
for the most part, humanoid monster girls are supposed to be as similar to humans as possible cause anti-logic reasons
I don't think you are supposed to think about monster girls through a lens of logic...it kinda is the exact opposite of the magic world standard that they usually live in, and the insect types are probably the least logical of those based on actual existing creatures (plant types come close, but they have an established fantasy creature archetype backing them up so they don't get full points...I would concede bacteria types would be even less logical if they existed, but I've never actually seen someone go THAT far in creating monster girls for their story world)
...that said if you did need a logic based reason, then it could be that it somehow otherwise benefits them (perhaps for survival purposes, rather than hunting purposes) to appear relatively human and they evolved to that shape, after all plenty of real insects evolved to mimic the shapes of either inanimate objects that would be ignored or dangerous ones that most animals wouldn't dare to actually eat