Because they aren't "pro-choice". That's a lie; a rhetorical dodge meant to make shredding innocent kids acceptable to mainstream America.
They're about as "pro-choice" on abortion as Ford Motor Company is "pro-choice" about your choice of automobile. Mothers who choose to let their babies live are, to them, potential profits lost. The so-called "pro-choice" people aren't "pro" that choice, at all. The only "choice" they favor is abortion.
They are pro-abortion, but use "pro-choice" to attract libertarians and those who are just confused because there is not and never has been a majority in the US in favor of abortion on demand as a means of birth control.
But IMHO, the whole abortion movement is built on lies, the first being that a fetus is not human but just "material." Ultrasound pictures expose this lie.