Metaphor doesn’t work in the opening of fantasy and science fiction.
Take a traditional work of fiction that opens:
Tom was a rock.
From this sentence, we know two things: Tom is a solid, reliable character, and the writer isn’t very imaginative.
Now that a fantasy novel that opens:
Tom was a rock.
In this case, perhaps Tom is an earth elemental. Or maybe we are reading something like a fable, with talking rocks, trees, and animals.
A science fiction novel that opens:
Tom was a rock.
means that Tom is a silicon-based lifeform, and we will be dealing with aliens (either just aliens or a mixture of humans and aliens).
The opening of fantasy and science fiction works establishes the world–thus, metaphor can’t really be part of that. Once the world is firmly established, then metaphor becomes workable because we the readers know what is and isn’t part of the world.