a screenplay is written for the express purpose of being produced for the screen (hence, the name)

you can read an unproduced screenplay and enjoy it, but there's still the sense that an unproduced screenplay is an unfinished product

a song is NOT written for the sole purpose of being recorded. people wrote songs long before there was any kind of recording technology. and even now, people write great songs and never record them. a song that hasn't been recorded is not unfinished or inferior to one that has

(in fact, some people even think that a song that has been recorded is a little inferior because it's been stripped of its vitality and frozen for all eternity)