So I know tampons have an expiration date and I used one from a box I had before I was pregnant its…

Probably not. For products like that, I think they do it to cover their asses, so if something did happen (which it likely wont) they cant be sued...

It's just due to the fact that they're not sterile. If they aren't kept in a cool, dry place they can grow bacteria and mold. And nobody wants that in their vagina!