@stbrandonsboat because the alternative is to grant personhood to a person who is within another person's body.
Which leads to the nonsense you have in the us with criminal charges for using drugs in pregnancy, etc.
And when does that personhood begin? Do you decide it's a person at conception? Before you even know you're pregnant? A huge proportion of pregnancies miscarry very early without the woman knowing she's pregnant, what about that?
You can't give equal personhood to a being that is wholly dependent on someone else. Because that would overrule a woman's right to bodily autonomy.
Eg imagine being pregnant, being diagnosed with cancer and being told you couldn't treat it because it would harm the baby.