
I used to feel the same as you. But the short answer here is because the law chooses to focus on the welfare of the child over either of the parents. It isn’t about being fair to the mother or the father. The mother and the father made the child. The child needs care. The mother and the father are the most reasonably responsible for its care.
I think it’s that it doesn’t take itself too seriously, while still remaining carefully in the world/rules they set.