The reason for that is to cover the cost of raising kids (in my extremely basic example). If the idea is to replace things like welfare, food stamps or whichever programs cover the cost of raising children, I think it's easiest to assume that everybody gets the same dollar amount (the de-facto average amount). There's a lot of ways to try to dice up the numbers (family size, married/unmarried, etc), but I think as a basic example, the numbers are fairly solid.