Content holders don't see their products as fungible
The copyright laws are stopping a decent TV experience and Apple are caught between content providers in the same way as everyone else. They've done what they can with universal search but Youview and Freeview plus have been trying similar with the UK providers and that's had similar results. They've got an app but all that does is accesses each sub-app (iplayer, All4 etc).
Amazon are not going to put content on a provider's box until they abandon FireTV and each cable company thinks they should be paid extra for losing their monopoly, making deals unrealistic.
Let me know when I can ask Siri to cache me the latest TV series that's not been licensed for my country. In the meantime I await the glut of Spectrum games being ported to AppleTV.