I would think there's people out there who do just have a limitless taste for all kinds of music, and really do enjoy listening to everything they possibly can.
But the people with ridiculous amounts of songs in their library; come on, there's no way you could possibly enjoy every one of those songs enough to justify having spent the money on them, what's the point other than having a "Mine is bigger" showoff?
For me, it is one of my favorite things to do. I have music playing all the time, whether at work, on the road, or at home. Music is always playing. I have been collecting media (LP and CD's) since the early 80's. I have over 1500 CD's and quite a few LP's. Not to mention the amount of free music I have downloaded from iTunes and Amazon. I have ripped all of my music to digital format.
I have a sonos whole house system set up. I have PLEX (some movies, but mainly for music) setup on all my mobile devices so I can stream. I fire up the apps and hit shuffle and away I go. Or if I am hunkering for something in particular, I search it out and play it.
According to iTunes it would take me quite a long time to listen to every single song, if I went one by one.
I like music, so it is easy for me to have that many songs and albums.