Because it's hard. There's implications of autonomy here. If an unborn baby is a life and has rights, which rights does it have? Is it an American Citizen? It must, at least, have personhood - what are the guardrails around that? Can I, for example, say that the unborn fetus is a person and is inside me, and is therefore bound by "stand your ground" laws, meaning I am free to kill it for self-defense, much like I would a grown person?

See, the Supreme Court never actually asked that question. As it stands, unborn babies are not people, they are not citizens. We, instead, said "fuck it this is hard to argue" and made exactly 1 exception for unborn fetuses - abortion.

If you want to argue an unborn fetus is alive, there are implications and consequences of that. Evidently, conservatives are too cowardly to address any of them. Ever. So, here we are, with our broken laws that make no sense.