Why do Mormons have so many kids?

Even though I've lived in Utah for more than 30 year, I'm still at loss to explain why Mormon families still have so many kids in an overpopulated planet that is brought to the brink of destruction because it's simply overcrowded.

A recent Pew Research Center study found that the average number of children ever born to Mormons is 3.4 in the United States. By comparison, the average number of children born to members of historically black Protestant churches is 2.5 followed by Catholics and evangelicals at 2.3.

While the Christian share of the U.S. population is dropping as the number of mainline Protestants and Catholics keeps on declining. Yet, that same study found that Mormons held steady. Their share of the rising U.S. population was 1.6 percent in 2014.

This still doesn't answer my question, though.

For one thing, I believe that the goal of large families is simply increasing the constituency and Mormon make it an unspoken commandment; historically, having so many kids has been an easy alternative to conversion or recruitment. Further, kids are so easy to brain-wash that they make faithfuls for life (probably a strong form of child abuse too...)

Then, there's that belief that families are eternal, and when people believe this, they tend to want more of it. Also, since they marry young, Mormons are quite fertile. Along the same lines, the Mormon Church proscribes it members from using birth control to reduce the size of their families, but this guideline doesn't seem to be followed too strictly as most LDS families stop at the “magic” number of 5 kids...

Finally, when it comes to feeding their large number of kids, Mormon have a large percentage of their members relying on a convenient state and federal assistance.

If you have additional answers, please, share them with us.