ทำไม subjective complement ถึงมี verb แต่ objective complement ถึงไม่ต้องมี verb ก็ได้คับ??

subjective complement
David is a student.
S      V       C S
Her daughters look lovely.
S             V       C S

objective complement
That boy called his dog Bravo.
     S       V       O       C O
He painted his baby's room blue.
S       V       O             C O

ใครเก่งอังกฤษช่วยชี้แนะด้วยคับ