Science fiction and fantasy do get celebrated for their literary value when they deserve it. A good recent example is Michael Chabon's The Amazing Adventures of Kavalier & Clay
. It won a little award called the Pulitzer Prize in 2001. I don't know how you can get much more celebrated than that. There are plenty other examples of celebrated SF and fantasy. Frankenstein
is usually regarded as a pretty important work of literature, for example.
Yes, there are literary snobs who look down on these two genres. But, truly great works of literature transcend their categories. People don't celebrate mainstream fiction just because it is that. It has to be good to get recognized for its literary value.