All the borg we've ever seen were assimilated humanoids. It wouldn't be a far leap to guess that the original borg species was humanoid. So perhaps it is a racial memory that keeps the borg in their physical bodies, instead of going to the nanite route.
yea thinking about the same. that they're subconsciously wanting to keep their biological side even though they are striving for perfection. they're unable to leave all the imperfections behind because subconsciously they can't. however, it doesn't explain why they're not using their tech or their biological potential to their most efficient and most improved. that's what i'm trying to understand. trying to understand their concept.