because giving birth is something incredibly powerful and amazing for women. the mother carries a baby in her womb for such a long amount of time which causes her to develop a strong, unbreakable bond with the baby. thats what my mother explained to me, its just such a strong feeling they have towards the baby, if you dont give birth, you wont ever be able to understand it.
that being said, if a woman and a man breaks up that doesnt mean she should stop him from seeing the child, thats just completely wrong and evil. no matter what happened in their relationship to cause them to break up, the father still has every right to be in the childs life too.