1. This assumes someone has actually A/B tested to see if this works. I would believe no one at Spotify has tried this.
2. What about if other music had lower royalty rates? Doesn't Spotify lose money, so wouldn't they want to pick cheap music?
I don't have specific knowledge but they probably try to optimize for retention. As such if they concluded that for example discovery is an important part of retention then switching artists might bring less listening time (which is not issue as long as you don't cancel) but more attachment to the platform.
Additionally the way they distribute royalties (as I understand it it's tied to the total of stream not your usage) might have some strange optimization. Like maybe they need to guarantee a minimum for the big player otherwise they might leave or in the opposite maybe they try to drive content away to pay them less and lower their influence.
Note: all of this is random speculation, I have no idea what happens just tried to think of possible use case where what user and Spotify want is not aligned.
If always playing songs from the same artists is what people wanted (e.g. lead to more overall listening time), Spotify would 100% do that. You can't pick what a radio station plays next after starting one of your songs, likewise Spotify gets to pick what their users prefer (visibly: not always songs from the exact same artist).