-As people grow old, they are economically dependent upon people younger than them.
-A growing population goes hand-in-hand with a growing economy. This tends to bring prosperity to many people.
And as a bit of a tangent: The argument that a shrinking population is good for the planet and is therefore better than a growing population feels to me like an extremely pessimistic argument, and the people that I personally know that embrace it have a much less human-centric moral intuition than I do. I put a lot lot lot more worth on human happiness than that of animals, and to me it seems that a cultural disconnect from the natural act of procreation implies an unhappiness that is rooted in how cultures are run nowadays.