or they are not schooled musicians so they can't adapt to actually writing something after they have exhausted their limited musical repertoire.
Note: there have been and will be lots of great musicians that can't read or write music but put out tons of great stuff but they are the exception. The schooled musician has a fantastic edge on the non schooled, at least in the kinds of music I like. They can draw on any type of music they wish because they know the language and become more well rounded, which shows in their music.
note: IMO.
which isn't worth a piss hole in the snow most days