 Why had the children never seen Earth?
They had grown sated with the sight of wonders—of multiple systems of giant stars, of nebulae that sprawled in empty flame across light years. It was a subdued excitement; men and women, they came and stood quietly gazing into the big vision screens that showed the oncoming star, and there were wide-eyed children who had been born in the ship and had never seen a planet. People pressed round the screens, peering into them as if with the naked eye they could pick out the atom of reflected light that was Earth, home. Earth's still there." And we don't know what changes have taken place on Earth.
 They were born while on Quest III.
 The ship didn’t have enough fuel to return to Earth.  Earth was destroyed before they were born.  They weren’t able to see through the portholes.  They were born while on Quest III.