If all the music you want to listen to is available via streaming services than great, but I have a bunch of MP3s that aren't (things ripped from old CDs, music from my friends, music from anti-streaming artists).
With that needlessly arrogant attitude you might be surprised to learn that there's lots of music, new and particularly old, rare material, that has never been available via streaming. Or might disappear one day due to ending licensing deals. I'd estimate that about 20-30% of my collection isn't available via streaming and that includes pieces I do not want to miss.
I understand that this isn't as important to everyone, but as a music enthusiast I refuse to rent my music collection, which I intend to build and maintain for decades to come.