How does a car seat expire?!? it doesn't make sense. what harm could it cause the baby? and y doesn't the other stuff expire, like the swing and bouncer

Wear and tear: The straps wear, age, and weaken. Those straps must be able to tolerate high levels of stress in the event of a severe crash. The manufacturer is not willing to certify that it will meet requirements after a certain period of time. If you choose to ignore the expiration date then the responsibility is yours. Swings and bouncers are less mission critical.

Shell issues/ tech: The plastic shell of seats begins to age when made.They often sit in the car with temperature range from < freezing to >120.This stress turns the plastic brittle, in a crash the straps rip through the weakened shell and baby can be ejected from the vehicle.Improved technology also places older seats in a lower quality category, not a good thought for a precious rider.