Because shipping a CD without jewel case without being tracked… I don’t know. I guess most buyers would not want their CDs sent way (the danger that the CD becomes scratched or just gets lost is enormous). Often times, when CDs are in a jewel case, the jewel case arrives cracked; you know of people are irked just because of scratched jewel cases. I imagine it would be worse if the CD gets scratched.
So usually, people want their packages tracked and their CDs in jewel cases and securely wrapped.