Art encapsulates a hidden pathway/ethic/system. People gather together around a transcendental object which adds something to their collective consciousness that unifies, heals, transforms, etc. Art is creation, is beyond ego and the cult of personality, which nowadays turns artists into go(o)ds that sell themselves on markets. This kills the sacred dimension of art.
Discussion
There are hundreds of years of esthetics theory, even thousands if we go back to the Greeks. Nobody knows, there are no accepted theories of it. It cannot be defined, this is the most clear thing we know about it. Because creating art is a transcendental phenomena.