It's very simple: for a myriad of many possible reasons, it is very common and very reasonable to believe that birth is the line between being a fetus and being a person. Before, it's a fetus, then it's a person.
Now, I am very much aware that this line between the two is contested, but, that then raises the issue of who gets to decide? Who even has the right to decide?
Again, I'm not looking for you to change your mind - that's just how the answer to your question goes. Any other questions?