Why would the Apple TV app play other streaming services? I know the Amazon Prime app supports some partner streaming services, but I don’t know of any other streamer that plays its competitors videos in their app? Every service requires using their app and only plays its own videos in that app.
The Apple TV (the device) has a “stuff this user watches” app (called Apple TV) which has a tiny subset of its features dedicated to AppleTV+ (the service).
Netflix refuses to participate in “stuff this user watches”, it would be trivial to do, but Neflix jealously guards its viewership numbers and I expect this is the main reason they don’t do it. That and… they’d rather you just browse Netflix and not watch other services.
The “stuff this user watches” app is very useful! I like it a lot, when I’m not watching Netflix stuff! It works with every service except Netflix!
But the moment the family shifts over to watching some Netflix show, it forces us out of the habit of using the TV app, and then we go back to the annoying “spend 90 seconds trying to find what we were watching on Hulu” experience, which is worse in every way.
To add some additional context to the other comment, the Apple TV App launched before the Apple TV+ streaming service. About 3 years before.
The point of the app was to consolidate all of your watching across various services to a single source. It could work with third party apps and can sign up for various streaming services from within the app. This is a major way it differentiates from the Amazon Prime app that you can still use the third party apps.
So it playing third party content was always the point of the app, Apple TV+ just fit nicely within that app. Less them being competitors and more just why it exists in the first place.
Netflix to my knowledge is the only one that is resistant to using this service, I have not seen any other not work with it. The app is very convenient for keeping up with what I watch across services.