 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." He said, with a touch of tolerant amusement, "What did you think might have happened to Earth? Someone switched off the lights, and a charmed whisper ran through the big room as the people saw each other's faces by the pale golden light of Earth's own Sun, mirrored and multiplied by the screens. And we don't know what changes have taken place on Earth. If success comes there, a century will have passed before we can return to Earth; friends, relatives, all the generation that launched the Quest ships will be long since dead. "We face the unpleasant realization that what was feared is probably true—that worlds such as the Sun's are a rare accident, and that we may complete our search without finding even one new Earth. Certainly the building of this ship and its two sisters, the immense expenditure of time and labor and energy stores that went into them, left Earth's economy drained and exhausted.
 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.