All the car seats for my kids were chiefly composed of plastics. I'm sure there is quite a high factor of safety for car seat design, but there are quite a few time and temperature related failure modes for plastics (i.e. getting brittle over time as the plasticizers outgas, UV exposure, etc.).

https://www.sintef.no/globalassets/project/ffs/dokumenter/se...

https://www.amazon.com/Plastics-Failure-Guide-2E-Prevention/...