The problem with those TV apps is DRM. All the major streaming services require that you either use a locked down platform (probably checking SafetyNet and more on Android TV) or settle for their browser UI which lacks dpad support and gets quality throttled to 1080p or lower.
Circumventing that DRM is possible, but no project at the scale of a platform like those would dare the both legal risk and support headache of making those circumventions (which are very liable to break) a core part of the OS.
Kodi (and distros using it like LibreELEC) exist for people who want a FOSS platform for using non DRM encumbered media with a TV remote interface.