Comment by nerdjon

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.