Some adjectives describe absolute conditions. Obvious examples include dead and pregnant. Some less-obvious examples are perfect and unique.
A person is either pregnant or not pregnant, dead or not dead. One person is not more pregnant than another person.
Similarly, something cannot be more perfect than something else. Perfect is perfect.
If something is unique, there is nothing exactly like it in the universe. Something cannot be "very unique" or "more unique." It is unique, or it isn't.