There's also the fact that in many poor nations, there simply isn't enough of a social safety net to take care of you in your old age. The surest form of insurance is to have as many kids as you can and count on some or all of them to support you when you're no longer able to work.

With SS, Medicare (in the US), etc. even though you won't live in luxury, you at least have some kind of income coming in in your retirement years.