God is not he, God is not she. God is God, who created male and female. God doesn't have human traits and cannot be fully understood in human comprehension.
One who is omnipresent, omnipotent, omniscient, needs no mate.
All creation receives love from God, and God receives love back from creation.
At least that's the version of the story I believe.
Now in saying what I said, others may see it differently, and it's fine for them if they do. But since you asked why the Christian God has no mate, that's the answer I have with my limited knowledge of God.