Because women in these countries don't have access to birth control, and/or it is banned in their country.

If you simply look at the data and the evidence, you will see that women usually do not voluntarily have a lot of children, as a tendency. High numbers of children almost always coincide with women who are essentially forced to become pregnant and force to carry the pregnancy to term, either through the law, their religion, or tribal traditions--or any combination therein.

Women who are given a choice almost never have more children than they can afford. The solution is to give women choices.