because it makes work fun.
yeah, it is distracting. i will pick songs, and read up on artists a bit while i'm working.
but it also makes work more enjoyable. it feels like i'm lightly working, living life and having a good time. it makes it feel less like i'm pushing myself, doing something arduous. it feels a little more like a treat and that i'm slipping into something comfy.
sometimes i'll turn off the music to focus but sometimes i'll leave it on to have a good time.
one of my friends described writing a document while watching Bojack Horseman. sure it took him an hour instead of 15 minutes, but wasn't the hour a lot more fun? and would he have written the document if he didn't have the show to coax him into it?
there's a productivity technique in here somewhere. see animedoro.
tangentially related, what if kids that were homeschooled had 80% more fun as kids, but 20% more worst outcomes in life. is that so bad?