TL;DR: The political arguments on either side of the pro-choice/pro-life debate have virtually nothing to do with religion, so why are the politics and people's positions on this issue so correlated with religious stance?
This is partially inspired by this other question.
I'm trying to keep my own personal politics out of this question, except as background to help clarify and provide context with. That said, there are two very different ways to phrase this question, and the one I picked for the title seemed to be the more neutral one:
Why do politics around abortion affiliate so heavily with religious status?
Of course, some of the information provided in response to the question linked above had to do with the US's somewhat religious nature, when compared to more secular countries. And indeed, demographics such as Muslims and Christians are far more likely to oppose abortion than some less religious demographics, and it is often preached in churches as a matter of faith.
But there's an aspect to this that kind of leaves me wondering: (Again...as background for the question!) I myself am a Protestant who is opposed to abortion, but I don't see much in the Bible that specifically comes right out and says: "Thou shalt not commit abortion." It (and here is where the other potential title for the question comes into play) seems to be just a simple matter of looking at photographs, learning about how DNA is inherited from both parents, etc., then believing that the zygote/embryo/whatever is a person. At that point, it's not so much Scripture, my pastor, my church, or anything else that's directly making me think there's any particular rule specifically against abortion itself; it's just that I (and many demographically similar people) view it simply as another case of a much broader category of wrongdoing - a broader category which most people around the world agree about, even if they do disagree on the specifics.
Now, when someone argues that abortion is not murder, I have heard arguments claiming that an embryo or whatever is currently a part of the mother, claims that it's not mentally developed enough to count as a person, considerations of what may happen to the mother during delivery or afterward, considerations of whether rape was involved, that sort of thing. But even then, we're still not debating this on religious grounds, but on scientific and sociological ones. My own answers regarding these (background, context) would probably generally not be articulated with a Bible, but with things like DNA and historical anecdotes.
So here's what I'm asking: Why is the pro-choice/pro-life debate so attracted to and affiliated with religious status? If we're all approaching this from primarily a scientific and sociological standpoint, what makes groups such as (for example) Christianity and Islam so different on this issue than other groups, and why would this be such a large reason for a more religious country's politics on this to look so different from a more secular one's?