Because that's what the Bible says.
Why do YOU limit God to a singular way to doing things?
God can do things any way that He likes and that's how He did it.... two roles in one Messiah. King of Israel and Head of the Body!
WE, the body of Christ, are complete in Him. But you already knew that.
Eph 1:9-12 (AKJV/PCE)
(1:9) Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself: (1:10) That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; [even] in him: (1:11) In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will: (1:12) That we should be to the praise of his glory, who first trusted in Christ.
It is hard for you to see the TWO-FOLD action going on there? Are you dumb enough to think that we live in the FULLNESS OF TIMES?
P.S. Your quote of my original post was terrible and a bit deceptive.