Struggling to find the paper with actual tests, but there was a separate statistical analysis backing this up, and here's a link to another paper confirming those results: https://docs.iza.org/dp8590.pdf
Because it's a huge chunk of the labs revenue, and there are other labs the companies would want to work with. Then the automakers who make up the rest of the labs business are now potentially liable for kids fitting without a car seat, instead of being able to transfer that liability to the car seat makers. What is the moral thing to do and what are you incentivized to do are very often opposite.
It just causes far less headaches for automakers to keep the existing laws mandating child safety seats, so the liability can be transferred to other companies that now have a reason to exist, and you have a way of feeling better by spending $500 on the fancy seat instead of 100 bucks on a cheap one that works just as well.
Struggling to find the paper with actual tests, but there was a separate statistical analysis backing this up, and here's a link to another paper confirming those results: https://docs.iza.org/dp8590.pdf