Same reason music and movies are sold in genres, I suppose.
And because people love labels. As far as I can tell there are no Americans in America - only African Americans, Chinese Americans, Hispanic Americans, Norwegian Americans, WASPs, Canadians (often referred to as God-damned Canadians for some reason), Jewish Americans, Arab Americans, Japanese Americans, Irish Americans and three guys descended from passengers on the Mayflower...
I think if you just say, "I am a diver," that is fine but it invites the question "what kind of diving do you do?"
Nobody minds much what label people give themselves. Except on the internet. The internet was made for pointless arguments like that.
