First, I don't see why Ayn is expected to not create fantastical characters in a work of fiction. Nobody would read it if it were just a mirror image of society.
Do you really not see the difference between Frodo Baggins and Dagny Taggart? Frodo is a fantastical character - he's a short-statured member of a race of hairy-footed little men who live in hills, have eleventy-first birthday parties and possess a strange resistance to magic. But despite all this, he's understandable as a person: he hopes and struggles, he gives up and sometimes he wins. The actions he takes are ones we could see ourselves taking, if we happened to be in his fantastical situation and under the stresses he's under.
Dagny Taggart is a fantastical caricature - she's a human, but not as we know it Jim. Everything she ever wanted sort of just happened to her, and she just does random insane shit because that's what the author needs her to do in order to move the plot along. It's really hard for an actual human being to identify with her, because she's the barest sketch of one.