I work for a company that has some periphery exposure to FDA-regulation, and as a result we have to do a lot of very similar scientific calculations to satisfy their archival records. A big part of why most medicines still expire (at least for medicines available over the counter) much earlier than they should is that a majority of the testing is done to determine the expiration date at a controlled temperature, pressure, and humidity.

They test how it degrades under stress to determine a maximum viability under perfectly stable conditions. They are not accounting for the typical medicine storage places: a bathroom cabinet where the temperature and humidity will vary wildly within a matter of minutes, a cool, damp fridge, or even the reading table next to your bed-- constantly exposed to UV light from the windows.

I'm relaying this secondhand from a former FDA contractor, but I can only surmise that for a company with the litagative risk level that any pharmaceutical entity incurs, the old adage of "Better Safe Than Sorry" is a major part of the story that's missing here.