1. Those are often semi-permanent installs

2. Those must be professionally installed

3. Those are pricey

4. Those are, quite by accident, illegal in North America

It's still impossible for many vehicles. Plus, slim fit car seats are expensive. Here's the cheapest:

https://www.amazon.com/Graco-SlimFit3-Forward-Highback-Kunni...

That is the problem. Death of the birthrates is by a thousand cuts. The more friction you put in society to having children, less of them you will have. There are thousands of small inconveniences of having children that accumulate.

In my country we have had in the last decades the following - car seats, prohibition small children to roam free, prohibition for kids under 12 to be left alone at home, the size of the city apartments has shrunk substantially - from two bedroom to one. The impossibility of stay at home parent. You can add to the list.