Because a fantasy story still has unwritten laws to follow.
And one of those laws is: If the protagonist has it easy, the story if bloody shoddy to begin with.
Superman sucks. He's a wussie, coward weakling. You know why? Because he's bloody superman. Hey; I'd be superman too, if I were bloody invulnerable, could fly (or jump very far), had the strength of twenty or more humans, and had, as my only weakness, a mineral that's not even native to the solar system I reside in.
Then the writers need to fabricate all sorts of stupendously idiotic reasons for that mineral to suddenly be bloody EVERYWHERE for no good reason, just to create the illusion that the protagonist is actually facing a threat rather than an antagonist who is barely even a nuisance to our 'hero.'
All sorts of people seem to assume that, in fantasy, anything goes. But the only 'fantasy' in which anything goes is the 'fantasy' written by people who lack imagination, and are only interested in shinies and sparklies.