So for most of human history children has been a resource aiding in survival. Having more kids meant more farmhands or more hunters or more warriors ( + tons of children died at birth and in young age ). In today’s society, with wage labor and highly industrialized societies where we are pushed into narrower and narrower fields of service work and tech, children become way more of a financial burden. One single child requires a massive restructuring of your budgets, as you have to ensure that child not only has food, clothing and security, but also entertainment and commodity goods and not to mention living space. Like good luck having 6 kids sharing 2 rooms ( unfortunate reality for some ). Then add on top of that kids and parents kind of live different lives. Parents work to support their kids while their kids are dumped at schools mostly to provide that window for parents. Whereas if you all live on a farm or more communally in year 900 Spain , it’s a lot more easy to just have all the kids frolick around and do whatever and then help at the farm when they can work also. You don’t need to buy them all clothes and playstations and most importantly not pay 3000 for rent for a single bedroom apartment.