Comment by TeMPOraL

From what I heard, in very poor countries children are often treated as investment. A child can start paying itself off by working as soon as even 5 years after being born. As a parent, you want someone to take care of you when you're old, and given high children mortality, you're better off making more of them in hope at least one survives to adulthood. Basically, those people are too poor to afford not having many children.