Because Superman fights a never-ending battle for truth, justice and the American way.
Because that's the way God made it.
And, most importantly...
Because growing up in Cuba, I took pleasure in reading my collection of Superman comics; they had been banned by the Communists as being too American. When my family was getting ready to leave, my Father made me destroy them so there would be no evidence for the thought police to keep us from emigrating to the USA.