Because the dirty secret is that "new" does not mean you are the first customer to open the item.

When you return an item to Amazon, one of the reasons you can select in the dropdown menu is "received used item" (or something similar).

It's disappointing that this scam is so pervasive they added UI to handle it.

And for most items that’s not too big of a deal. A returned textbook or toaster can be re-sold after it’s been opened without much fear that it’s been tampered with but I’m not sure the same can be said for a hard drive.

Or maybe it’s more like a bottle of medicine. Even if it’s exceedingly unlikely that it’s been tampered with I’m not going to risk it if the manufacturer’s seal is broken. It’s just not worth it.

Textbooks are actually a bad example, now that an awful lot of them have one-time codes linked to online materials (homework, etc.). Sure, the contents of the book may not be "used up", but that doesn't mean it's necessarily "good-as-new".