I'd strongly argue you want to deplete reserves of foreign immigrants that want to come fill in any population deficit you have before you start subsidizing reproduction. Humanity as a whole is still growing, that population growth is largely limited to poorer nations, and as such it just doesn't make financial sense for nations to be paying people to have kids (and bear the associated stresses, complications, and expenses from having often full time laborers also burdened with trying to raise a family) when there are billions in poverty that immigration could uplift.

It also has historical context. Colonization around the world happened on the back of permissive immigration - when early America needed labor it didn't pay every family to have more kids, it brought the poor over from Europe with the promise of opportunity. And while the investments needed to make a worker productive are much higher today than back then it applies to both a natural born or immigrant citizen all the same.

A lot of it is cultural though. The unrealistic expectations heaped on most western working class peoples to both have a successful career and somehow produce a well adjusted functional progeny feels to me like the same kind of fantastical unrealism of the titular American Dream - where everyone could have two cars and a separated house anywhere, drive everywhere, and that scaling that up to hundreds of millions was both practical and optimal. In practice, no, very few members of the working poor are skilled in parenting enough to somehow magically balance 40 hours a week between two parents on subsistence income with raising children effectively. Perpetuating that lie causes real harm to everyone involved. Its the same kind of doublethink that keeps global warming off the agenda as a legitimate threat - the need to protect culturally established norms even if they cause real harm to perpetuate.