Patty this is an amazing question.
I apologize ahead of time, because my answer will be honest and given as my own opinion.
The reason for my probably unpopular answer is from over a century of gathering data, being involved and paying attention to what is happening in and on my globe our only habitat.
I have noticed there are greedy boys who think the world is their very own sandbox, To them, the rest of us are cheap replaceable plastic play soldiers and Indians - easily tossed aside.
Lost joining in the rock cycle of geological transportation.
Sometimes when bad actors are called out on their misdeeds they will behave. Left unfettered and unregulated they are monsters who need some time with a peach tree switch.
When settlers came to spread their seed over everything they could see with the naked eye. They grasped hold of unrealistic parcels of Earth.
Some scripture and some smooth talking leadership convinced these new criminal immigrants that they should take what land their eyes could see, and eradicate what hampered the perfect vision of Manifest Destiny.
They needed to populate their land so they could farm, create their own kingdoms and subdue Earth. The animals and People who looked like the Earth were to be subdued as the Creatures who inhabited this "New World." In order to do this they needed a group of people they could rely on to help build this corporation. It would start from their own loins. If there were not enough being born, new wombs were purchased. Slaves were bought.
It no longer requires a large population for a family to survive.
Resources are not shared fairly and this is causing a scarcity of food and the ability for people to simply clothe and house themselves.
In our society if you own certain items you are considered "in." Elementary and Junior High kids who do not have "i" something are not considered in at all. Having fewer children make it possible to do more for them materially.
Regarding "Cheaper by the Dozen" it is true that having a larger family would assure them generational wealth.
In large families or tribes who share ethically resources seem to grow exponentially when everyone is contributing.
Less is more where love is the chief operating engine.