 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
A scream of brakes, the splash into icy waters, a long descent into alkaline depths ... Dazzled, and befuddled by his own rash speed, Ned Vince had only swift young reflexes to rely on to avoid a fearful, telescoping collision. His car hit the white-painted wooden rail sideways, crashed through, tumbled down a steep slope, struck a huge boulder, bounced up a little, and arced outward, falling as gracefully as a swan-diver toward the inky waters of the Pit, fifty feet beneath.... Ned Vince was still dimly conscious when that black, quiet pool geysered around him in a mighty splash. Movement was slower now, as he began to sink, trapped inside his wrecked car. Nothing that he could imagine could mean doom more certainly than this. The edges of that almost bottomless pool were caked with a rim of white—for the water, on which dead birds so often floated, was surcharged with alkali. Time, the Brother of Death, and the Father of Change, seemed to wait.... The icy breeze tore tiny puffs of dust from grotesque, angling drifts of soil, nearly waterless for eons. A launching catapult for the last space ships built by the gods in exodus, perhaps it was—half a million years ago. Glacial ages, war, decadence, disease, and a final scattering of those ultimate superhumans to newer worlds in other solar systems, had done that. Loy Chuk had flown his geological expedition out from the far lowlands to the east, out from the city of Kar-Rah. He could think and dream and invent, and the civilization of his kind was already far beyond that of the ancient Twentieth Century. The dry mud that had encased it like an airtight coffin, had by now been chipped away by the tiny investigators; but soiled clothing still clung to it, after perhaps a million years. A mineral saturation that had held time and change in stasis. A perfect preservative for organic tissue, aided probably during most of those passing eras by desert dryness. Not the star-conquering demi-gods, but the ancestral stock that had built the first machines on Earth, and in the early Twenty-first Century, the first interplanetary rockets. A strange accident, happening in a legendary antiquity, had aided them in their quest for knowledge. The chant of triumph ended, while instruments flicked in his tiny hands. The final instrument he used to test the mummy, looked like a miniature stereoscope, with complicated details.
 No one has figured out time travel.
 The government of Kar-Rah turned down Loy Chuk's request to use time travel.  There is no such thing as time travel.  No one has figured out time travel.  Humans took the secrets of time travel with them when they left Earth.