Me: This sounds terrible… but ok, let’s give it a go.
Google: Now that you have all your music in the cloud, wouldn’t it be nice if you paid us monthly for access to a lot more music?
Me: No.
Google: I see you switched to another app while watching a YouTube video. If you paid us extra, you could keep playing that in the background!
Me: First, why would I ever want that? It’s bad enough YouTube now keeps playing videos in a little thumbnail when I try to exit them. Second, why are you charging a monthly fee for a feature that ought to just come with your app?
Google: Hey, how about a free trial of our subscription service?
Me: No.
Google: Hey, how about we ask you every day if you want a free trial to our subscription service?
Me: Still no.
Google: Ok, I tell you what. How about we shut down Google Play Music, literally the only built in MP3 player, and then if you want to keep listening to music on your phone, you pay us monthly?
Me: Buys an iPhone.
It makes sense - Google can't run YouTube without ads. Ad buyers, which have ads in video form, don't want to run ads when the user isn't looking at the content nor able to easily click on their link to convert them to a paying customer (plus google never gets paid as the user probably won't switch to the app just to click the ad). They either do this or ask advertisers to make ads specifically for audio-only streams (which still makes it hard to drive conversions), but then they'd have to charge advertisers for impressions which Google has very rarely done.
I definitely prefer music videos over plain audio streams.
It's good enough for a lot of people. I pay for YouTube Premium, so I get YouTube music (formerly Google Play Music) included, and it works well enough that I'm not going to pay for a separate music app.