Actually the GAN systems do have a way to check if art is beautiful. This is why they can produce novel results via experiments. So yeah, such system have a sliver of an idea of what they're creating.

The problem is not in discrimination at all, but in generation of novel and correct output. (grammatically and syntactically) Discerning correct syntax and grammar has eluded all AI so far. (Even specialized translation systems are worse than babies at it.)