> as a user, I would find it too "buried" behind Safari
How is this any different than music playing on say Google Play Music on my iPhone? I listen to music from YouTube, iTunes, iHeartRadio, Google Play Music, Pandora, and other apps occasionally. Why don't I feel it's buried in those situations? Because of the simple reason that Apple allows only one source of audio on iOS at any point of time (including Safari), which can be controlled from Control Center. Control Center also displays the app, and tapping on it, takes me straight to the app.
There is no reason a tab on Safari couldn't similarly queue up a playlist, and if I want to get back to it, I go to control center, and tap the safari icon which takes me to Safari and the tab playing the music.
Now I don't think Apple prevents this out of malicious reasons. It's likely a vestige of their older restriction mechanisms which they haven't gotten around to changing because most people do in fact just create apps for things like music playlists.
But I seriously doubt there is any good UI reason to prevent a tab in Safari from queuing a playlist, any more than there is for preventing native app from doing so.