They are exceptional by design. If they weren't amazing swordsmen, or magic users, or highly intelligent, or highly determined, they'd be dying at the first hurdle in the story - which is equally ridiculous for a novel. The alternative for underpowered fantasy characters is either a non-perilous story or a lot of "deus ex machina" moments where the character survives by "random" chance. Neither of which are particularly interesting.
Narrative imperative does have some demands on story characters, but it's all down to the quality of the writing as to whether you can suspend your disbelief.