Because they want to support Safari and Homepods generally.

> Music you buy from all services is DRM free and music you stream is only in the apps and have no need to support FairPlay

Every significant music subscription service has web based playback, which will use a DRM system to protect those streams, and will use a multi-DRM system to support multiple browsers. Safari doesn't support Widevine, so if the service works in Safari that means the service also serves FairPlay Streaming licenses.

> Even the ability to play movies bought in one store and being available in another store as a purchased item is supported via MoviesAnywhere integration and not FairPlay.

Very difficult to see what this has to do with the topic at hand, but Fairplay is used to stream those titles to devices that support it. MoviesAnywhere is back end, not distribution. Fairplay has been ported to non-Apple devices too, at least the lower levels.