Probably because they're branding it as "Apple Music", not "Music (by) Apple".

On an iPhone the app is just called Music (similarly Google Maps is labelled just Maps on Android). This is exactly as weird as google maps domain being googlemaps.google.com

The iPhone Music app can play music from Apple Music, amongst others. It is not synonymous for Apple Music. You can still have your local library without subscribing to Apple Music.

On checking further, this is because music.apple.com redirects you to the Music app on iPhone. So they use a different domain for blogs.

I've never seen googlemaps.google.com?

Sorry. Should’ve been clear. It’s not the case, it would’ve been weird if it was

Also there is a record company called Apple Records that has a litigious history with Apple, it could be not worth the potential trouble.

That redirects to the non-classical music app page already.