They also thought that citizen's morale would break when a few bombs landed. WW II proved that it's more resilient, or perhaps resignation is stronger than giving up?
Maybe the Americans were still remembering the first World War, in which the Germans already had low morale by the time the USA joined the conflict. The USA joined in the thick of WWII when the Germans/Axis were doing considerably better. A prevailing (but ahistorical) view among Germans after WWI was that they had surrendered prematurely in 1918, so that might have also led to a cultural disdain towards surrender.