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Even without capitalism you need production, and children used to be part of that. Back then you would have as many kids as you could so that they could run your farm.
I’m not defending the current system, but profit isn’t the only reason the birthrate is declining in so many countries.
Japanese workers get more days off than American workers.
Japanese on average don’t work longer than Americans (2017)
Don’t get me wrong, they have a crazy work culture, but it’s worse in the USA.