There are good reasons for that, from their point of view. Children are their hope, children are their only security, and, because they have little control over their own fertility, even more children come to them than they aim for. At some point the accumulated consequences of their powerlessness will surpass the Earth's limits - if indeed those limits have not already been surpassed. The consequences will not be visited only upon the poor. In this economically and ecologically interconnected world, overpopulation anywhere affects everyone.
Population growth is the most undiscussed problem on the planet.
No one talks about population and no one does anything about population because no one knows what to do about population. Or, more accurately, everyone pretends not to know what to do. We pretend for many reasons. The nations of the poor South have trouble discussing the problem directly because they entangle it with their ethnic hostilities and their old resentments toward colonizers and oppressors, so as not to have to admit their own corruptions and inequities. The rich North entangles the problem with its unresolved attitudes about sex, religion, and abortion, and therefore fails to share fertility-control technology.