My understanding is that’s the lifespan that the packaging can reasonably be expected to remain sterile. After a number of years, the packaging may start to degrade and no longer be air tight, which means it’s no longer sterile.
However, these aren’t typically sterile materials, I realized as I was writing. I honestly don’t know, maybe someone else can chime in
Edit: after reading the article, they’re said to be no longer as safe/protective for the person wearing them, but I’m very curious how a date was established, and who determined that it was only good for a few years