Why do reserves go out of date?

The manufacturer’s service life is the best guidance there is

It would be ideal if we could test reserves like we can paragliders to be able to conclude they remain servicable, but unfortunately that’s not possible. Given that the pilot’s life may depend on the speed and reliability of deployment, we follow the BHPA guidance to observe the manufacturer’s guidance on emergency parachute service life.

If you would like to know more, read this article