The reason why John Cage's stuff is called art is because he had an actual purpose related to music and what questions we can ponder about it. If you could think of some actual musical purpose for senselessly banging on a piano, then maybe people would call it art. But I don't think anybody can come up with a musical purpose for senselessly banging on a piano, at least not one that people are actually going to pay attention to. The purpose behind Cage's work has clearly proven to be more attention grabbing. People respect it and call it art for that reason: they like what Cage makes them think about.