Pre-agriculture, everyone's job #1 was getting food on the table. Post-agriculture, you had a surplus of food, so people began to specialize. Economic and market forces started to become more sophisticated, as people were secure enough that daily survival wasn't an issue.
So economically speaking, before the advent of modern farm machinery, what did a farmer need to prosper? Answer: Labor.
Where did this labor come from? Answer: Children.
Farm families are historically speaking very large. Much larger than what a hunter/gatherer could possible support.
A woman might have 12-15 children, and since you have a food supply, many of them will survive to adulthood. So if you have 15 term pregnancies, starting at age 15, you're talking about being almost continuously pregnant until age 30.
In primitive surroundings, pregnancy is deadly. Today, the lifetime risk of maternal death in sub-Saharan Africa is 1/16. (vs. 1/2800 in the developed world).