The thing that makes some art 'good' isn't just a straightforward display of technical virtuosity (which alone almost always guarantees 'bad' art) but involves a delicate, ineffable balance between skill and seduction.
The skill part is easily assessed and quantified (and even reliably cultivated given enough time) but the rest of the equation remains radically mysterious and magical. That's why there are so few good artists around and why we pay them so much money and attention.