Because she already collects a massive amount of utility associated with raising her own children. The economics of paying people to raise their own children doesn't make sense; the externality allocations are completely wrong. I'm sure you can imagine some of the perverse incentives that come into play.
A woman benefits from raising her own kids (with an associated opportunity cost in employment availability), and the father also benefits from her raising the kids (without the same opportunity cost), so the rational economic strategy is for the father to defray some of the opportunity costs to the mother. This arrangement has existed for at least thousands of years and is called "marriage".