Why do you include Amazon Music but not Apple Music when I select music apps?
No clue; my guess is that Apple wants too much money to allow their music to be streamed.
Maybe we will get it now that Apple has separated iTunes music in a separate app.
It has only been very, very recently that Apple has had any interest in playing well with others with things like the Apple TV app for other devices. And Comcast probably requires a substantial amount of testing before rolling out anything new, so it may just be a matter of time now. If there are disagreements it might be on how subscriptions or remote controls are handled. On some xfinity apps (like Qello) you have to buy the subscription through xfinity - even if you already had one elsewhere. And apps that you can 'cast' to from your phone on other devices, you can't with the xfinity app. If things like that concern you, you might want to buy your own device to handle internet apps.