like others have said, its because his yin, the energy that creates form from nothingness , was sealed away, and considering bijuu are just big ass lumps of chakra with consnciousness, his form has to be maintained by naruto in bm mode
with regards to the 8 tail situation vs pain, i think when a bijuu is taking over a body the laws that govern the chakra balance between bijuu and jin are different, and its my theory that kurama was using naruto's yin chakras to reconstitute his physical form as he was slowly taking over, i dont think he can do that now, even though their chakras are melded, because like others have said theyre new at using this bond, it will take time, but thats just imo