I think there is some ego involved in the parents of those types of broods, especially in the fathers. Like look at me the stud!!! And then the poor mother takes up the slack with all the work.
There was a family in my mother's parish who had 17 children. They were the poster family for anti-abortion/choice movement in the the whole city. They wore it like a badge of honor. In the meantime, the poor kids suffered. They didn't get the attention and care they needed. The father didn't even work, he was out on disability, so the church donated food and clothing to them all the time. And the mother just kept popping 'em out!
And believe me, dad was no stud muffin, but I think he thought he was.
Those Duggars are a goofy brood. But at least the kids look cleaner and are smiling more than the brood that I knew. Who knows what goes on behind closed doors.