Are original xbox cases just getting harder to find nowadays?
I believe there are licensing issues.
Many cases carry the brand-logos (xbox, playstation, etc.) and are made after patented designs from Microsoft, Sony or Nintendo.
Amaray is bound by whatever license agreement they have with these companies.
They cannot just mass produce and offer original cases however they like.

Maybe Microsoft does not want them to sell replacement cases seperately. Or maybe the license fee for xbox cases is too high.
That is probably why they produce non-original cases, that have the same size, but a slightly different design, colour and no copyrighted logos.