"radio" in this case refers more to 'cheap equipment played in a noisy environment'. Back in the 70s ~ 80s that meant car speakers playing over road noise. Now it's cheap ear bud or tinny bluetooth xyz playing over road noise / transit / coffee house ambient noise. It's not how the music gets to you, it's the environment you (most people, that is) hear it in. And on top of that, for most of the audience, it's background noise. So to catch your attention, it gets mastered as 'loud' as possible. Because the next / previous tracks in the playlist are. And so it goes.