Well we had an entire fantasy module at my University English course so Fantasy isn't entirely ignored by critics but you have to realise that truly great literature transcends a simply good story and makes some sort of comment about the nature of humanity, genre or literature itself etc. The writers the OP lists are spinners of good stories but I don't think they actually do much to advance literature or our understanding of ourselves.
I personally love a good fantasy novel. Wheel of Time and The Malazan Book of the Fallen and Song of Ice and Fire all rate as some of my favourite pieces of writing ever but do they really contribute much to the progress of literature as an art? Not really if we're being honest. Maybe some interesting additions and manipulation of the fantasy genre but not really the stuff you set kids to study in school. Alasdair Gray's Lannark on the other hand is widely studied and considered literary even though half of its narrative occurs in an entirely fantasy realm which is even further removed from reality than the series I mentioned because he reflects on the nature of society and the Scottish National identity through radically exagerated characters and situations made possible by a fantasy setting.
I personally love a good fantasy novel. Wheel of Time and The Malazan Book of the Fallen and Song of Ice and Fire all rate as some of my favourite pieces of writing ever but do they really contribute much to the progress of literature as an art? Not really if we're being honest. Maybe some interesting additions and manipulation of the fantasy genre but not really the stuff you set kids to study in school. Alasdair Gray's Lannark on the other hand is widely studied and considered literary even though half of its narrative occurs in an entirely fantasy realm which is even further removed from reality than the series I mentioned because he reflects on the nature of society and the Scottish National identity through radically exagerated characters and situations made possible by a fantasy setting.