NASA can't afford it with the budgets and programs it has worked out with the president and Congress, especially those aimed at moving human explorers beyond low-Earth orbit.
In addition, according to former NASA administrator Michael Griffin, while technicians went over the orbiters with a fine-toothed comb to ensure they were ready to return to service after the Columbia accident in 2003, the fleet's accident rate through 2005 implied a 1-in-8 chance of losing another crew if the fleet's operations were extended.
