Well, countries with higher birthrates have a third option that is essentially negligible in those with lower birthrates, which is not even making it to adulthood. Effectively still less children end up becoming productive members of society. And together with that, due to less available social services, often a goal of having children survive is so they can take care of the parent when they're older.

As soon as infant mortality becomes a non-factor, birthrates decline drastically as well. And since children are no longer largely seen as a "life assurance" for when parents are older, and the society's demands for productive members is higher as well, the focus really does shift to the quality of the life and the two types of reasons to have kids are harder to compare. But even among developed nations you can see differences in fertility rates.

PS. Scandinavia doesn't have the lowest birth rates, they actually have fairly typical birth rates for more developed regions.

Maybe I'm reading into this wrong, but I think the interpretation of fertility statistics may be underestimating/overlooking how much rape and sexual violence contributes to the high fertility rates we're seeing in impoverished countries struggling with widespread violence.

Countries like the ones in Scandinavia have lower rape statistics and access to abortion which could explain a lot about those numbers and why they are the way they are. Again, it's a just hypothesis, but one worth mentioning I think.