When I buy a new product, I’m paying for everything that comes inside the shrink wrap, case included. Once you open that case, the case is used. It is no longer in brand new mint condition.
If GameStop had a reputation for taking care of their open boxes and ensuring that they were in as close to mint condition as possible, I probably wouldn’t care. But they don’t. Every time this happened to me, I got a scuffed up case covered in hard to remove stickers. In some cases, you end up like OP and pay the full new price for a bare disc with no original case or manual. The fact that you are OK with that is astounding.
Why are you so eager to defend shitty behavior that only GameStop engages in? Are you just completely oblivious to the fact that many people who still buy physical games want to keep the packaging in good condition?