How did they even come to such a perspective? There are all kinds of physical handicaps in fiction.
Raistlin had a mysterious uncurable ailment imposed by Par-Salian.
Albrech has to forsake love to attain the Rheingold.
Several gods and heroes are missing various limbs.
And blindness? Daredevil. Tiresias. Any number of blind kung-fu masters.
Sometimes they're afflictions that are paid as a price for powers, sometimes their curses, sometimes their obstacles that heroes overcome. But disabled people have been all over fantasy literature for millenia.