Hard to know what goes on behind the scenes in any particular case but I think this "discrimination" goes both ways.

For content like that to be a first-class citizen on iOS/tvOS, the providers (Spotify, etc) would need to expose the relevant metadata, APIs, etc to Apple.

I know that on Apple TV, there were a lot of providers who balked at providing that information, preventing Apple from fully implementing relevantly simple features like system-wide content search.

I certainly don't blame the providers, because they wanted to maintain more branding and autonomy rather than being a faceless Yet Another Source of Content on AppleTV, because at that point they'd be essentially faceless interchangeable.