The correct answer is that Japan assumed US planes could not get close without detection. It was impossible to launch land-based bombers from ground airfields since Japan had taken over most of the Pacific, including the Philippines. Aircraft carriers would have had to get too close to Japan to launch bombs. So the Americans decided to launch land-based bombers from aircraft carriers. The Japanese actually were warned of an impending bomber group, but assumed that they would have to get much nearer to attack, so they were surprised when the Americans launched from farther out than should have been possible.
