That's because of iOS was never good at inter app communication (Android's version of intents) and that made Siri a worse experience for users and ultimately doomed it in the assistant wars.

I don't think Apple management sat down and thought, right, how do we kneecap competing music streaming providers? Okay lets build Apple Music support for Siri but not Spotify.

It was just that no iOS apps had deep Siri integration and they've been one-offing implementations for common usecases like timers, alarms etc. for the longest time. I assume someone got a project funded by the Music org to go one off the Apple Music usecase as well and here we are.