You can thank the people at the SFI, who used test results from 1966, to set the replacement date at 2 year intervals.
SFI Presentation:
http://www.sfifoundation.com/wp-cont...%2012-2014.pdf
The Europeans however, must not being getting payoffs from the seat belt manufacturers, because their test results show a much different result when using the exact same seat belt material. Either that or DuPont only sends the "good stuff" to Europe and the US only gets what fell off the truck.
European Test Report:
https://www.easa.europa.eu/system/fi...nal_5-2010.pdf