This is so that you don't kill a human by mistake.
If I'm wrong, no one dies by not killing a "fetus" who is somehow not a human.
If you're wrong, we've murdered 50 million people in this country.
You use 3 terms here. Human, fetus, and person. There are, however, some quirks.
A fetus is a human, no real question about it.
A fetus, however, isn't a person. At least 'not yet'. You look at historical societies, you can see ones that wouldn't ascribe a baby as being a person, they wouldn't even give it a name, until it survived it's 1-3rd birthday. Many, many, only bother upon birth.
Persons, whether they're human, alien, AI, or even uplift, are what deserve our protection.
I want abortion to be rare, yes, but in general I'd rather a woman carry a child to term because she wants to, not because she's forced to.