A pregnant woman gets hit by a drunk driver. She survives, the unborn baby dies. Can the drunk driver be charged with murder? If he is, can he argue that it can't be murder because a fetus is not a human being and has no right to life?
It would lead to a huge contradictions in the law. You can't have the same things categorized as completely opposite things at the same time - a life and not a life.
Of course the resolution is easy, but unpalatable to the left as they rely heavily on misleading labelling of things: the unborn baby is a baby, a human life, in both cases but in the case of abortion someone is claiming they have the right to kill that life.
This is what they don't want to do - admit that they want to kill babies. You see it on this thread - repeated attempts to claim everything is just a fetus, when it clearly is not.