The narrator isn’t omniscient; he only tells us things from certain characters’ points of view. I agree that it is suspcious that neither Dumbledore nor Harry think of this. But in fact neither has any reason to suspect that Voldemort is still alive, while we (having read the original series) do. (Also, Dumbledore was really bad about this sort of thing in canon.)
The narrator isn’t omniscient; he only tells us things from certain characters’ points of view. I agree that it is suspcious that neither Dumbledore nor Harry think of this. But in fact neither has any reason to suspect that Voldemort is still alive, while we (having read the original series) do. (Also, Dumbledore was really bad about this sort of thing in canon.)