That's possible, but it would probably cause more headaches than it's worth. There's 2 big hurdles there, one is it would really confuse a lot of people (even if we spelled it out perfectly for them), the 2nd hurdle is that we would have to manually go into every single order and add a separate item for the case. It's a lot of orders and would take a large amount of time, and since it would have to be done manually it'd be prone to errors. So while possible, probably not worth it in the long run.
Another thing to note is that the cases actually provide an extra layer of protection in shipping. It's not the end of the world if a case gets damaged (there's usually plenty of extras), but there's only soo many books to go around.