Feel free to insert "potential" before any instance of child, mother, or father in this post, it's implied but I didn't want to be redundant
Because both scenarios deal with a born child that is not living by using a woman's bodily resources as his own. When the child is born, if either parent walks away, the parent is responsible (unless they both agree to put it up for adoption, obviously). When the child is not born, the person who is allowing the child to use her body to develop calls the shots, because it is her body. Just like if my best friend/child/brother/whatever was dying because my boyfriend hit him with a car and needed a kidney and my boyfriend was compatible, I still would not be able to legally force him to donate his kidney. Although the situation was his fault. Morally, he should. Legally, it's his body and I have no right to dictate how he uses it.
Of course I agree that if both parents can, they should be involved in the decision. The question is how that would work out legally. The father should not have the legal power to force the mother to carry a child to term, because that would be the father dictating to the mother how to use her own body and telling her that she must donate her body to the child. Morally, the father, if she's not a victim of incest or rape, should have a very important say in the decision. Legally, there is no way to implement that without there being a scenario where the father forces the mother to use her body the way he wants her to.
I think there's a moral/legal disconnect here. I'm not arguing that all women should use abortion as birth control or that all women should make their choices without consulting the fathers of the children. What I am saying is that legally, it is impossible to make a law like this that will not legally allow men to dictate how women use their bodies, which is a fundamental right. And just as I have explained numerous times, there is no other situation in which because you did something legal, you are legally obligated to use your body a certain way or allow someone else to use your body a certain way. If there are any counter-examples you can come up with, they would be welcome. The only time you would lose bodily autonomy in the US (speaking from these laws since I'm obviously not well versed in others) is either if you broke the law or you are a vegetable, in which case bodily autonomy is taken from you only because you are incapable of making decisions.