Why does only the father have to pay child support?

This is not true; every state requires both parents to provide child support. This is often referred to as "the equality of duty." It may seem as though that the father is solely responsible for child support, since he often is the non-custodial parent. The mother is often the custodial parent, and her "child support" typically manifests itself in other ways: providing a home for the child, and providing for day-to-day expenses that are not manifested in a physical object like a check. Her support is given directly to the children.

If the father is the custodial parent, the mother will be assigned a court order to pay child support to the father. This is becoming more common as more fathers become custodial or joint custodial parents.