My suspicion is it's the financialization of everything driven by fiance capitalism and enabled by the huge twentieth century population boom.
I know that sounds edgy but explaining it.
With the population surging you could just flat out ignore the need to have children and create the next generation. So the priority then became amassing wealth. As long as the working classes were still behaving like they were living under malthusian conditions that worked. But now the working classes have caught on and are either trying to acquire capital wealth or they're trying to just hang on.
So yeah the working class knows what economic reality they are living under and acting accordingly. Having few children in order to acquire wealth and funnel it to the smaller next generation. The tell this is true is that it's happening everywhere. And the longer a place has been operating where childhood mortality is low and you can acquire financial wealth the lower the birthrate. With the caveat that some places are more culturally amenable to that.