I pay for YouTube Music and premium gets thrown in for free. Why? It's mostly momentum.
I used Groove back when that was a thing, and when it folded I could migrate my playlists to YouTube so it was easy and than $10 a month. Now it's $30 but it's still like buying a cd a month. I am slowly de-googling and some month will stop using the service. It just hasn't been a top priority.