> I guess the main reason is that women were/are not empowered enough to decide how many kids they should have. Another major factor is lack of availability/knowledge of contraceptives.
The Hesperian's "Where There Is No Doctor"  offered another explaination - in some parts of the world people are too poor to not have many children. A child needs few years to grow, and then it can work for the family. This makes children an investment that returns itself with profit after few years, and it is good for the family to get many of them.