Humans are not singular organisms, we're a colony species. We are evolved for (and thrive in) social groups. These social groups are not just voluntary associations, but are deeply rooted traditions - whether that's based in family, faith, or nation.
The problem with Modernity is that the systems and structures that humans have built to ensure continued human flourishing within these deeply rooted social bonds have grown far beyond their original scale. You can blame capitalism, the state, whatever, it doesn't really matter. What's important is that as the state or capital grow (and they have to, or be killed by others which do), they must necessarily tear down the old order; debride the old traditions which hinder its continued growth.
The end-state of this, which we can see best in places like East Asia, or California, is society in which humans are interchangeable chess pieces. People are atomized from any received community or tradition. Thus, reproduction is viewed not as a natural stage of life — an essential component in the ordered tradition in which they live their life — but as an object of luxury consumption. Very luxury! Which is why people in these regions tend to stick to one kid, maybe two (and if you walk around Seoul, you'll see plenty of dogs in strollers: much cheaper!)
In societies which haven't yet reached that end state, we see the same effect as the forces of Modernity inexorably creep in: religion wanes, tradition falters, birthrates fall. Be it Salt Lake or Suzhou, Tehran or Tokyo, we see this pattern in every place wherever Modernity is made manifest.