I grew up during the age of peak CD sales, and I still only bought like ten audio CDs in my life, all of them on the secondary market. I essentially didn't spend any money on music before Spotify came along. Now I spend 15 EUR every freaking month.
And it's not just piracy, although it is that, too. Before Spotify came along, I listened to music radio, and I also listened to a lot of free amateur music. And that was at a time when distribution was a challenge; now any artist can create a Bandcamp (or whatever) page in seconds and give away their work.
The entry barrier for making music is just incredibly low. I'm sure only gifted individuals can make exceptionally good music. But most music isn't exceptionally good and loads of people can make music that's good enough to have on in the background, i.e. what Spotify is being used for the vast majority of the time.