They don't want to think about whether a particular song or artist is on a particular CD.
They want to play one of anything, as the mood strikes them.
Generally, they're probably fine with leasing (cheaper) access to (more) music, versus (more expensively) owning (less) music. Ownership just doesn't confer enough utility to make up the price gap.
I'm as anti never-ownership-trend as you'll find, but it's hard to fault Spotify for "winning", given the huge disparity in user utility.