Because any type of art is usually not measured by a scorecard.
Of course, you could apply scorecards to it, if you'd like. For example, you could measure a musician's worth based on album sales or critical reviews or Grammy awards. But by those merits, you'd have to concede that someone like Celine Dion sings some of the "best" songs ever. I, for one, wouldn't concede that.
But really, it's not just music or art that's subjective. Even in sports, where statistics play a much more direct role, you have some people who favor certain players over others just because ... because of their style, their personality, their versatility, etc. It's really all subjective when you get down to it.