The way the pension systems are set up, you're donating your children's labor to the pension system as a whole. This means any investment into your children is purely an expense. Meanwhile if you don't have children, you can save money for retirement and let other people's children work for you.

This means children are a net positive to society as a whole, but a net negative to the parents raising them.

But that's not relevant to couples choice to get pregnant. I've never heard a parent say they had kids so they can pay into Social Security.