My father-in-law flew fighters off of aircraft carriers in the Pacific during WW2. He explained once that flattops were not actually that big. Often times, multiple waves of planes were in use at the same time. Torpedo bombers might be being retrieved at the same time that dive bombers were being launched. The flight deck was not large enough to have all of the planes landing and taking off at the same time. So some planes might be catapulted for launch when, at the same time, other planes might be landing.