@BillRM,
Because he has acted as the father. Like I said, if you assume the role as parent the law sees you as the parent. And, like I said, those are the laws that would have allowed you to adopt your step children if they indeed had no contact with their father and they considered you to be their father.
Maybe they should test all kids at birth. I really don't care.
What about couples that use sperm donors? Should that father later be allowed to opt out of his responsibility to the child?
Who do you expect to pay for these children if not the person who has parented them?