The USA simply cannot afford to take the risk of the plane suddenly failing or developing mechanical issues in-flight. For that reason, it's probably the best-maintained plane in the world. A specialist team ensures that it's always in immaculate condition.
When it's not on duty, the plane (and its twin) are under armed guard at the Andrews Air Force Base. You can't even get close to it without top security clearance. Even the fuel has to be handled by special agents!