For political systems, I can look at how happy people are, how healthy people are, how affluent they are, how much economic progress there is, how much scientif

For political systems, I can look at how happy people are, how healthy people are, how affluent they are, how much economic progress there is, how much scientific progress there is, equality, stability, as well as apply whatever theories of morality I like (e.g. utilitarianism). There is a value judgement as to how I weigh those, or which ones I care about, but I can evaluate it in some way. People might argue about the merits of a meritocracy versus a democracy, but broadly speaking, everyone will agree that a progress democracy is better than an exploitative, violent, oppressive dictatorship. The quality of art, in contrast, is fully in the eye of the beholder. If you'd like to see how this manifests in practice, it's worth looking at Soviet Russia, the French Revolution, or many democracies which failed in practice, despite what were, on the surface, plausible ideas. This contrasts with literature, where there is no objective metric, and much of the time, someone says "that's great, now let's go even further" to an idea as good as North Korean style communism.