It's really not. Without some form of rules and expectation, then no art has value. It's impossible to shock people or make them think if EVERYTHING counts. To say nothing of general standards of appeal and subjective means of definition.
It's also intensely more creative to have to find how to express yourself within an understandable structure than it is to smear shit on the wall and call it art. I find a piece that shows a solid grasp of technique and actual skill in expression to be far, far more valuable than a vague, unstructured blot of color that only holds expression because someone told you it has meaning, or because you made a meaning up on observing it.
That's something that the impressionists got right. Even in going against the traditional rules, they still kept enough relation to structured art, that you didn't have to have the subject explained to you. That there was still composition present and a story to be told.
As opposed to modern high art, where literal shit in a jar gets called stunning because the artist acts hoity-toity and the rest of the art world doesn't want to seem behind the times. Where actual skilled artists get no recognition because they don't network well enough, because their work doesn't make an unintelligible 'statement' that is 90% interpretation from the viewer, as opposed to an actual expression of the artist.
I hope I'm coherent enough to be understood here. In the end, art is subjective, but ultimately, if everything is art, than is ANYTHING really artistic and creative?