He has to adopt her separately

A child from previous marriage does not become the child of the new spouse automatically. The new spouse has to officially adopt the child.

Without adoption, when the new spouse dies, the child is not in the inheritance chain of the new spouse. This is critical when if both parents died at the same time (and without any other child), only part of the parent's property goes to the child, while the new spouse's portion of the property goes to his relatives.

The child's biological father would be always on hook for child support.

Most people argue about this is reinventing the wheel. This issue has been resolved in law several hundred years ago. Family law applies to kings, lords, capitalists, and poor peasants, have some respect before you criticize it.