system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do baby car seats expire after 5 years?

If it's due to the plastic deteriorating over time, I would think that someone would have created an affordable replacement part by now.
gpt
Plastic does deteriorate over time, especially when exposed to heat or cold, which a vehicle can easily reach 100+ degrees or below freezing.

Also, when a vehicle is in an accident, the impact can create stress on the plastic that creates small fractures that aren't noticeable but the integrity is compromised. 

For manufacturers, that's just a lot of reliability so by limiting it to a certain time period, it reduces that liability.