Why do they expire? So the manufacturer can sell more car seats. Think about it. They build a car seat to make it through a crash. They have to be built sturdy so they will survive a crash. By building it sturdy, it will last a LONG, LONG, TIME ! ! ! The company can't stay in business if it doesn't make more car seats, so they put an expiration date on them. Then paranoid parents will throw them away and they can sell another one, and stay in business.
GM, Ford, and all of the other car comapnies had the capability to make a long lived engine. Before 1965, they had no incentive to make the engine running parts (cylinders, pistons, crankshafts, cam shafts, etc.) harder so they would not wear out and they would last longer. (Its called tempering.) Before 1965, it was a rare engine that would last longer than 50,000 miles. There was an entire industry in the US that rebuilt engines. Then Congress passed the emissions laws. The end result was that to clean up emissions the manufacturers had to increase the combustion temperatures of engines. That required that the cylinder walls and pistons and almost all of the metal parts in an engine had to survive higher temperatures. The result was that the cylinders and all of the other metal parts had to have much much better tempering. Now a 100,000 mile engine is not uncommon. I have two vehicles that will probably go to 200,000 miles. The car companies would love to put an expiration date on new cars, but the public wouldn't stand for that.
When they discovered King Tut's tomb, they found wheat stored in the tomb. According to our government (The same one that lets car seat manufacturers set expiration dates) the wheat expired CENTURIES ( ! ! ! ) before it was discovered. Scientists took that wheat and found that it was not only edible, but was growable. They used that wheat to improve the drought tolerance and disease resistance of modern wheat.
The same "superior intelligence" found in our government officials that said the wheat in King Tut's tomb was no good and should be thrown away declared that your car seats will expire.
If you believe what these government officials say, I'll make you a real good deal on the part of the Brooklyn Bridge that I own.
Good luck to you and yours.