Let's put it this way: droughts don't last forever. Good times come, bad times come. You know that you're going to get old some day, and the only person likely to take care of you at that point is the person you raised and took care of - he/she owes you, by long custom. Problem is, you also know the bad times are likely to come and take half your children. Now, how do you ensure that at least one child survives to take care of you, when at least half or more might not survive the next drought or war or famine or whatever? One guess, and it doesn't involve listening to some half-assed Westerner who keeps promising you aid and then coming up with only half what he's promised.

Let's put it another way: in China, where the earthquake struck and brought down schools, killing children, a lot of families who dutifully obeyed China's one-child limit found themselves suddenly - childless. No second child waiting in the wings to step into the shoes of the first, no future generation to see after their needs - unless they want to try a second start at it ten or so years later than the norm and be prepared to be still raising a child ten or so years after their peers.

The cure for overpopulation, oddly enough, is wealth and security. When people feel their children are secure, they have fewer children and put more effort into the few they have. When people feel their children are likely to be lost, they have more children and put less effort into each one in an effort to ensure that at least some reach adulthood. The children are not the problem dragging down impoverished societies - to the contrary, they tend to be the first victims when things get bad. As the society pulls itself gradually up, and the calamities grow fewer and farther between, the number of children goes down and their childly fate tends to get brighter.

When we tell people in the third world to reduce the number of children, a lot of them hear it as, "We would like you and your line to please die out so the rest of us can have an easier time of it."
If YOU were told that your region's future success depended on you having only one or two kids and taking the risk of ending life absolutely alone and without help when you hit your declining years, you would probably tell that person to stuff his aid where the sun doesn't shine and not to go telling people to court genetic suicide for the benefit of strangers.