It’s because we have taken positions that are logically very strong, but emotionally very weak. (And that’s both sides.) The reason we have done this is because the scientific evidence does not really support what we believe, so we have to get creative. Scientifically, a zygote is its own thing. It’s not a part of the mother. It resides within the mother. It has its own DNA and if we were to invent an artificial womb, it would eventually learn how to type on a keyboard and make political arguments on Facebook. This differs from say a tumor that if we put it in a suitable environment it might grow a tooth, but that tooth would not register for a political party. Science says human development is a continuum. We constantly change and there is no real point at which a zygote flips a switch and becomes ‘human being.’ A 12 week fetus and a three month old baby or a three year old toddler or a 13 year old adolescent are all just the same thing at different developmental stages. They may have different abilities, but there is no fundamental difference between them. When we talk about fetuses then, we have to invent a concept we call ‘personhood.’ This is a philosophical or religious concept, so that means that we fight about it. We are not able to detect any mystical aura that suddenly inhabits a fetus and flips the switch from ‘non-person’ to ‘person.’ Most of the things that we can measure regarding ‘personhood’ that people try to use as hard and fast dividing lines are shady and can apply to humans that most of us can agree are people at different stages of life (A paralyzed person that can’t feel pain as an example does not become a non-person, or as mentioned above, someone in a coma doesn’t become a non-person or a 1 month old baby which basically is a fetus that is living outside of a womb isn’t a non-person.)
What happened in the US is that abortion was decided by courts instead of by legislatures. This is really great if you want to ‘win,’ but not so great if you believe in nuance. Since abortion is a subject with a lot of nuance, this court victory basically screwed up the debate for 50 years. What happened on the pro-choice side is that instead of making arguments to a legislature which can inject nuance into a law about perhaps young girls in bad situations and how people were suffering from unsafe abortions (The arguments that were used pre-Roe v. Wade.) The rhetoric had to shift to ‘fundamental human right.’ Well, that’s stupid, because most people then and now probably don’t see abortion as a fundamental human right. Most of us acknowledge that there are appropriate and inappropriate times to have an abortion. “The dad is black and I don’t think a black baby goes with most of my wardrobe.” or “I don’t like girls and the baby is a girl” probably bad reasons. “The fetus tested positive for ‘I’m in pain every second of my existence disease.’” or “I’m starving to death and live on the streets and a homeless guy raped me.” probably good reasons. What happens though is that the court decision forced us to make the argument a ‘rights issue’ instead of a ‘public good issue.’ A ‘public good issue’ would let us say, “Hey, a 40 week old fetus, ain’t a fetus. It’s a baby and you’d better have a damn good reason for wanting to kill it.” while at the same time saying 'You poor thing, you were raped and it’s not right or in the public interest to have you carry a baby to term and co-parent it with your rapist." A rights issue though makes us say, “This is a fundamental human right guaranteed by the Constitution that shall not be infringed.” and it doesn’t give a rip whether it’s a 12 year old rape victim who can’t comprehend what’s even happening, or a 25 year old who thinks condoms and birth control suck and her parents keep paying for abortion as birth control. The courts did realize that what they were saying ended up being pretty gross, so they inserted a viability clause (Which is itself the dumbest thing in the world. When the artificial womb exists-and it’s coming soon-does this fundamental human right just disappear? Not so fundamental then, is it?) Human rights rhetoric forces us to say, “Why does a woman lose her right to choose just because the fetus looks like a baby?” and there’s not a good answer for that. The wingnuts then say ‘There is no good answer! Kill em all!’ but the reality is that most people, even on the pro-choice side recognize that there is a difference between a zygote and a 40 week old fetus and aborting a 40-week-old is pretty abhorrent. (To be fair, the pro-life wingnuts have the same issue in regards to rape. ‘A person’s a person no matter how small.’ forces some abhorrent things itself.)
So, to answer your question. A 40 week old fetus is a child. We all know it. Let’s not pretend differently for the sake of our rhetoric. 40 week abortions shouldn’t be legal because we don’t kill children. It’s gross and my culture sees such as immoral. I sympathize with the women who didn’t realize they were pregnant, but, we don’t kill children.
To be fair, I have always identified as pro-life, but I’m not in favor of making abortion illegal. I recognize that there are times when abortion is the best option and criminalizing something that someone does out of lack of options to me is like criminalizing stealing bread to feed your family (which I recognize is still stealing and currently criminalized, but should it be? I digress.) I do think that more regulation than ‘It’s the Wild West.’ is appropriate and that abortion should be seen as an extreme measure and not a common one. I have a problem when 1/3 of all pregnancies in New York end up in abortions. That’s not a few people scared and making mistakes. That’s cultural normalcy and that to me isn’t OK. In my ideal world, the laws would be the same, but birth control would be free, cheap and easy and culturally an unwanted pregnancy would be greeted with “What is in the best interests of this child?” rather than “What time does the clinic open?”