There is a fed guideline that is interpreted state by state with slight differences. The main reason is a doctrine based on maintaining the status quo division of time. If the mother is stay-at-home, she automatically has more time with the kids and the guidelines are to maintain that status quo; the dad has to fight an uphill battle. The money is divided according to the custodial time, regardless of who earns it. The result is that the default option of working less and earning less, regardless of who does which, produces the most benefit, while the primary wage earner gets reduced to a wage slave. Obviously there are other factors as well.
There is a fed guideline that is interpreted state by state with slight differences. The main reason is a doctrine based on maintaining the status quo division of time. If the mother is stay-at-home, she automatically has more time with the kids and the guidelines are to maintain that status quo; the dad has to fight an uphill battle. The money is divided according to the custodial time, regardless of who earns it. The result is that the default option of working less and earning less, regardless of who does which, produces the most benefit, while the primary wage earner gets reduced to a wage slave. Obviously there are other factors as well.