A lot of this ignorance is because of the education system there deliberately either whitewashes or skims over WW2, aside from focusing on being the victim of the atomic bombings. The origins of the war and its details are usually covered in cursory detail if at all, and then there is sometimes the outright revisionist or denialist stance. At best there might be the superficial acknowledgement of making a mistake but with the implied question of "Why is everyone else still so hung up over it?" stance, all the way to the revisionist claim that they were liberators fighting against white colonialism and that the native populations tragically "misunderstood" and fought against their liberators. Younger Japanese are thus kept ignorant and then react defensively whenever Japan is criticized.
The Japanese news is almost entirely focused on domestic news with very little international news reporting, except again in the most cursory details. There is as a result no real updating of stereotypes about China so many are still stuck with ridiculously outdated stereotypes similar to how some Americans think China is just like a larger impoverished drab looking North Korea. As an example, think of the character Chun-li and the background of her stages in the Street Fighter franchise. They are essentially all either portraying an impoverished or backward looking image with ancient/traditional Chinese styled architecture. The implied message is clear: "poor, backward, stuck in the past". They never portray high tech or modern development. At best they show a Hong Kong movie styled street with the stereotypical neon signage, so still not high tech or cutting edge, with maybe an undercurrent of danger or unsavoriness.