"The Bible never talks about abortion" Maybe not. But that doesn't mean the Church has never taught that abortion isn't a sin. Christianity didn't come from the Bible. the Bible came out of Christianity. While the Bible is the inerrant, inspired word of that (the Bible doesn't say that either. That is a teaching from Tradition) Christian life, morals and teaching were in full swing before there was a canonized bible. Hundreds of years beforehand, in fact. So how did Christians know how to be Christians? How did they know what to believe, how to behave, how to practice their faith? They learned these things from the apostolic teaching of the Church, through oral tradition and written letters.
So the idea that the Bible is a handbook on Christian belief is flawed. Christianity is rooted in the scriptures, but it is broader than the scriptures. Where does the Bible say that it is the inspired, inerrant word of God? It doesn't. That teaching comes from Tradition. Where does the Bible say that pornography or drug use is a sin? It doesn't. Are we to argue, therefor, that such things are morally neutral?
Abortion similarly is not explicitly addressed in the Bible. Does that mean it was never considered wrong, or that the Church never had an opinion about it until the 1970s? No!
Here are some quotes from early Christian teaching that demonstrate what the Christian teaching on abortion was, long before there was a canonized Bible.
"You shall not murder a child by abortion nor kill one who has been born."
The Didache [The Teaching Of The Twelve Apostles] c. 80-140 A.D.
"He struck the womb of his wife with his heel and hurried an abortion, thereby causing parricide."
St. Cyprian of Carthage, "Epistle 52 To Cornelius," c. 251 A.D.
"A woman who deliberately destroys a fetus is answerable for murder. And any fine distinction as to its being completely formed or unformed is not admissible amongst us.”
St. Basil the Great, "Epistle 138," c. 375 A.D.
"The rich women, to avoid dividing the inheritance among many, kill their own fetus in the womb and with murderous juices (Chemical abortion) extinguish in the genital chamber their children."
-St. Ambrose "On the Hexaemeron," c. 386 A.D.
"To destroy the fetus 'is something worse than murder.' The one who does this 'does not take away life that has already been born, but prevents it from being born.'"
St.John Chrysostom "Homilies on Romans," c. 391 A.D.
These, and other examples demonstrate that the teaching of the Church has been consistent in upholding the dignity and sacredness of human life even from the womb. "That's not in the Bible" just doesn't cut it. Efforts to use the Bible to justify abortion, or to suggest that abortion reflects "Christian values" as Taylor Swift and others have foolishly attempted, are even more desperate and ridiculous.