Because with each kid love is not just added, it's multiplied. And the world needs more love.

With one child, the pressure to "do something worthwhile with your life" is way too intense on that child and those parents. And the world needs less pressure.

Also, on a very practical level, your one child may choose or not be able to procreate. And the world needs all old farts like me to have grandkids on which to dote.

(Speaking as a father of five and gramps of three!)