I'd like to think this was part of some master plan to lull the Japanese into a false sense of security, but the facts suggest gross negligence and ineptitude on the part of the leadership of the defense forces. But allow me to digress. Historical events require context. Wars, revolutions, country music, and a thousand other examples of the ugliness humankind continues to subject itself to are not conceived in a vacuum. I suppose Hideyoshi's disllusions of grandeur with respect to Korea and China might also be conveniently overlooked, along with the other textbook follies, to ease the cognitive dissonance of contemporary Japan.
Nobody likes to see chinks in the armor.
The actual decision to invade was influenced in large part by the more than 100 years of civil strife on the archipelago predating the assault. From the mid-1400s on, the political cohesion of Japan disintegrated into petty infighting among a collection of regional chiefs, or daimyo , who were ensconced in their own castles and protected by their own armies. These feudal land barons squabbled over anything. The feuds between families were even more ghastly and puerile. Killing, looting, raping, burning family photo albums, these petty despots went after each other with reckless abandon.