Because the Holy Spirit calls her mother of Jesus.
And around we go!
I mean, when we say "Jesus," how many people are we talking about here? Are Jesus and God two people? Jesus is God, One Person, One Divine Person, one of the Trinity, existing before time even existed, co-eternal with the Father and the Holy Spirit.
At a point in time, approx 2000 year ago, He was born in Bethlehem. Who gave this Person birth? Mary. She conceived and gave birth to a person. She did not make Him, or create Him, or originate Him. She birthed Him.
Would I make it a little clearer if I called her the "birthmother"? I'm sure you wouldn't deny that. I don't think any Christian could.