That's an easy question to answer. The leaders of the Japanese military believed the U.S. was soft, and unwilling to commit to victory in a nasty fight. Yamamoto desperately tried to convince them otherwise saying "any U.S. surrender would require an invasion of the U.S., and the surrender would have to be signed in the White House", and "the Americans are a just and righteous people, they will not be defeated". He also told them "I can guarantee you that I can run wild for about 6 months in the Pacific, and after that I can promise nothing." It was almost prophetic, Pearl Harbor to Midway was almost exactly 6 months. Granted, Midway was a victory achieved as much through intelligence as anything else, but 6 months was all they got.