I think it is also about economy. Children need to be feed, clothed, schooled a place to live and someone to take care of them. since wages are low, houses (and everything else) are expensive and there is not paid maternity leave, many people can't afford it.

After all, there are women who have children while being single or unmarried. and there are couples who marry after having children.

The key issue is "can you afford to have as many children as you want to?" Because many people who would have two children can't. At least while giving such children a decent living standard. Things like one on one time, having their own room, going to a decent school, etc. Sure, you can make children share a room, inherit clothes, used food stamps, and made the oldest prioritize taking care of the smallest instead of their own education and lives. But many people don't want that for their children.

I think it is a change in values and about how much do you value individual childs. I mean, do you value that your older daughter can go to university or would you preffer that instead of study she is available to take care of her sibbling even if that means that she would never get that chance? Have you asked such daughter what she wants for herself? If she says that she can't take care of her sibbling because she has an exam, do you get angry? If you rely on older children to take care of the others and to make end meet, then you are passing the cost of having such children to your older childs and can't really afford it. Not really fair.