
There was a panel at Minicon some years ago - IIRC, the title was
something like "Science Fiction and Fantasy: Instantiating the
Metaphor". Someone (maybe one of the panelists?) raised an interesting
point - a number of things that are exclusively metaphorical in
mainstream fiction can easily be literal truths in SF&F. (For example,
my wife described a character as a silk-dressed cobra - the first
question asked about the passage was 'is she a snake or a human?')
Since there is that problem, the metaphors, similes, and similar
constructs in SF&F are either absent, or much larger or more
subtle. From what I can tell, this drives the academic nuts - part of
their world-view is that they provide the explanation of what the
author was saying by pointing out all these constructs, and explaining
what they mean. And in SF&F it's (mostly) just not there! The
whole story (or big blocks of it) may have meaning or illumination
outside the story proper, but it's not there at the nit-picky detail
level that academics love.