Yeah, that's because they're designed to absorb the damage into internal stress and deformation to dissipate the force of the crash.
That force and momentum has to go somewhere and it's better to absorb it into the car seat and consider it a consumable than to transmit that force into the child it's meant to protect. The same thing applies for helmets. A motorcycle helmet is a consumable item and should be discarded after a crash as the internal foam structure will be deformed and could fail to properly absorb the impact forces in a subsequent crash.
That force and momentum has to go somewhere and it's better to absorb it into the car seat and consider it a consumable than to transmit that force into the child it's meant to protect. The same thing applies for helmets. A motorcycle helmet is a consumable item and should be discarded after a crash as the internal foam structure will be deformed and could fail to properly absorb the impact forces in a subsequent crash.