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/...

This is part of the plan to force people to buy new. They make it illegal to sell any kids stuff older than a year or two.

To be fair, how do you test this - as a system - effectively. The consequences of failure on a child seat are not like, say, a stand mixer. That being said, I suspect it has more to do with the manufacturer attempting to limit their liability window. It's wasteful right, but in a litigious world, I get it.

Start crash testing expired seats and compare results to new ones of the same make/model. They expire relatively quickly, so they would not be difficult to find. For example, our Graco Keyfit 30 expires after 6 years.

I think you’d have to mandate it legislatively though. There are no incentives for seat manufacturers to do this of their own volition. The quick expiration works in their favor on every front...

Chicco Keyfit 30

Not quite the same thing (adult seats vs car seats, and it's possible they were just faulty from the factory rather than expired), but the last time I was rear-ended the seats snapped behind me. Quite the experience really -- to stop you from going forward you have seat belts, air bags, and all kinds of fancy engineering. Inertia is a cruel mistress when the car moves forward out from under you though, and the only things ensuring your forward motion are a couple of broken seats (thankfully there weren't any passengers or children in car seats for me to smash into) and the 2-ton metal projectile poking through the ass end of your car.

Some tests have found that brand new seats break when loaded near their stated limits. I think it is reasonable to say that plastic weakened due to the relatively harsh thermal and UV conditions in a car would fare worse.

https://www.consumerreports.org/toddler-booster-seats/child-...

I don't think a seat will get any UV exposure in a car, UV does not pass through glass

Glass blocks most, but not all UV [0]. And people open their windows sometimes.

[0]: https://pubmed.ncbi.nlm.nih.gov/12925188/

Regulatory Capture https://en.wikipedia.org/wiki/Regulatory_capture

It isn't the exception; it is the rule. Bodies that govern an industry are _inevitably_ captured by the industry they govern, because that industry has the absolute highest incentive to write the rules it has to follow.