Why are custodial parents not legally required to provide financially for their children?

I know three women who have primary custody of their children, where the fathers have visitation or modified visitation and pay child support. The fathers are, by law, required to have a job and pay for their children. Or, legally required to pay a certain amount even if they don’t have a job, which often becomes a catch-22. The mothers are not. None of these women that I know work, they are stay at home moms, and are supported by their current husbands. From a legal (and literal) perspective, they pay 0% of their childrens’ financial support.

If the courts do not factor any income outside of the custodial and non-custodial parents into financial support for the children, why is the custodial parent not legally required to pay anything at all?

I’m not talking about the moral aspect, I’m asking about the legal side. If a custodial parent’s parents or new spouse or random stranger off the streets gives them $1 million a month, and the courts do not factor that in as income for the children when determining how much support a n/c parent will pay – does it count as a contribution from the custodial parent? Should it?

From a different perspective: should household income be factored in (and adjusted as necessary) when calculating financial support for children? If dad has primary custody and lives in a mansion with his fabulously successful new wife who supports him, and mom has visitation while working as a waitress, should she have to pay him 40% of her income in child support, or should household living conditions factor in at some point?