The normal procedure is to set some of the tests aside when they first come off the production line. Typically every six months, you run a new study to validate that the tests you set aside still work. If they do, you can file an amendment to your approval to extend the expiration date. If this weren't allowed, you'd have to wait to bring your test on the market for two years or however long you wanted the shelf life to be.
The generous version is that it's only feasible for a company to make a guarantee of efficacy to a point and the cynical version is that people will buy more as long as the expiration is some economically tolerable length. The .MIL has enough to make the efficacy tests worthwhile.
https://www.fda.gov/emergency-preparedness-and-response/mcm-...
According to the FDA, the manufacturers are doing ongoing stability testing, and as they find the tests are still effective after the date they initially estimated, they can contact the FDA to share their results and request that the FDA authorize a longer shelf life. This actually costs the manufacturer money since they'd be paid when people replaced the "expired" product they threw away, but they're doing it anyway because not everything is a weird conspiracy (and maybe also because the most shelf stable products have an advantage when it comes to what people order for their stockpiles)