Well, just off the top of my head - the seminal works in the fantasy genre owe an awful lot to the likes of Tolkien and Lewis who relied heavily on the Judeo Christian mythology for their archetypical characters and, on top of that, were informing on societies with a different relationship with native peoples. While the "myth of the noble savage" you describe might, along with Early Modernists, possibly share roots in Victorian and Romanticist literature (as well as the Middle English epic), Americans were still coming to reckon with the systematic abuse of Native Americans at the same time as fantasy was in its formative stage. Whereas elves are portrayed as noble, but aloof - perhaps complicit in their own victimization - Native Americans are often portrayed as noble wounded animals - not responsible for the demise of their culture, per se, but not necessarily capable of adapting to invading white societies. There is a persistent and insidious problem with American Literature having difficulty in humanizing Native Americans in relation to their interactions with white settlers which isn't reflected in the fantasy genre.