They're largely subsistence farmers with high infant mortality and little to no automation which accounts for about 2 billion people. They need children to work the fields and it makes sense to have a lot of children when the labor turn around time is five years and up to a third of them will be dead by then.

The adults pay the "fixed costs" to keep the farm running so each additional child produces more in labor than they consume in resources. They're too poor to hire other adults for labor because they have their own "fixed costs" that are much higher than a child in the family.