Probably because it is more intuitive, emotive, and immediate.
I can sit with an instrument and play. Just play. I can go on for hours without a word-type thought going through my head. I can fall asleep playing. And I can play right up to the point where I fall asleep still holding the instrument.
I can't do that with my writing. Writing is hard work for me, not at all like music. Writing wears me out. So does reading.
An obvious question: is it like this for all writers? Or, is writing easier for some writers than is playing music? (Assuming both talents reside in the same person to begin with).
If that is a possibility that has a real world extension, then do those writers find writing to be more intuitive, emotive, and immediate than music?
Perhaps I should ask some writers.