The reason is that their drugs were stored under ideal conditions. Not many of us truly can keep our drugs stored in a cool, dry place without being moved for years on end. You probably store some drugs in your bathroom, where they get moist when you take a shower or bath. Or you may store some drugs in your kitchen, where the stove gets hot, or on the windowsill, where the sun hits them. Under these circumstances, it is not likely that drugs stored will become outright unsafe, but they could easily lose potency, which means they won't be as effective as they should be.
Health is too important to chance.
You want your drugs to work as they should. That's why it's so important to store drugs properly and discard them when the expiration date says to. You probably keep that cotton in the container. When you get the medicine bottle home and open it, throw that cotton away. The cotton is inserted to protect the pills from rattling around while you're taking them home.