Raising multiple children in western society is really hard. In many (most?) cases you need to move away from family (parents) so there’s limited support from the community or family. There’s limited childcare available, and on top of that kids are expensive. Often both partners are working (or want to get back to work as soon as possible). So if you can satisfy your procreative desires with one child, you just do that.

Compare this to (totally unjust) prior social norms where people didn’t move as far as they do now. Meaning parents could help out. Where it was common to have one non-working partner (so more available time to look after kids).

We’ve created a modern society where raising one child is super-stressful. Why would any sane person have two? I predict that as more societies transform toward the western model we’ll see a massive population decline.