Because He became incarnate.Jesus Christ even refers to his Father as “my God.” (John 20:17; Rev 3:12) whilst he was still on earth and even after his ascension to heaven. How does God have a God in heaven?
Jeremiah 32:26-27 KJV
26) Then came the word of the LORD unto Jeremiah, saying,
27) Behold, I am the LORD, the God of all flesh: is there any thing too hard for me?
John 1:1-2 KJV
1) In the beginning was the Word, and the Word was with God, and the Word was God.
2) The same was in the beginning with God.
John 1:14 KJV
14) And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Philippians 2:5-8 KJV
5) Let this mind be in you, which was also in Christ Jesus:
6) Who, being in the form of God, thought it not robbery to be equal with God:
7) But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:
8) And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.
Jesus, God the Son, took on flesh, and all that went with it.
Much love!