Because some crafts are more sacred than others. Making a painting is more sacred than smearing my shit in the Barns and Nobel bathroom stall, although arguably less fun.

Who decides that? We do, collectively. Why do we have that power? Because we define art. Why do only humans have this power? Because art is an innately human thing, so we get to decide.

I don't know what you mean by this. The same effect can be felt in other forms of art.