Because there are three eternally co-existent members of the godhead. The Father, the Son, and the Holy Spirit -- just like 1 John 5:7 tells us in the King James Bible. The very first word for God in the Genesis 1:1 is the Hebrew word Elohim, which is a plural word. And there are many more examples of the triune godhead. I don't believe in the Catholic concept of the Trinity, but I do believe in a Trinity. All three were present at Jesus' baptism -- God the Father spoke from heaven, and the Holy Spirit descended like a dove upon Jesus the Son. These three are One -- One in mind and character and purpose. But three in personhood. The Father is in the Son, and the Son is in the Father. But they are still distinct individuals who are One. I look at it this way -- the Father is so immense, so infinite, that He cannot be contained in a human body completely. But of Him there could, came here as the man Jesus Christ. So all the fullness of the godhead bodily dwells in Him. Yet, the Father is so much more vast that He cannot be solely contained in any one vessel. So there is still more of Him out there, so Jesus can still have a Father God.