What I like about rock songs is that you can put arbitrary content in them. Like good books, they have no blind spots. And indeed there's all kinds of bands with all kinds of messages.
Other music like Blues is more like narrow genre books, like detective fiction. You know what you'll find in a detective book and where the borders are. Challenge these borders and you're on post-modernist grounds, outside the genre.
Of course, rock used to be rooted in some things, just like sf&f used to be rooted in some things. But it turned out that using the method you can uproot it and tell about anything you want.