In my opinion it is because they did not really partake in many big battles.
Two fires of equal output, one inches away from your foot, the other several feet away from the same foot. You will respond to the closer fire since it affects you most directly. Now consider the same two fires a tall wall separating them, and you can only see one of the fires. Both fires exist and burn as much fuel, but which one will you notice?
Perhaps this a case of the old saying, "Out of sight, out of mind." How many reporters were covering the war in China? The reports that were submitted probably got less press from other powers, particularly western ones. After all, who cares about what is happening so far away when we've got issues right here in our own back yard.
One fictional book was written by a British survivor of the Japanese occupation of China. Empire of the Sun was a fictional novel, there's some real history in there as it was based on J. G. Ballard's own experience as he grew up in the Shanghai International Settlement during Japanese occupation.
The question provokes a plethora of thoughts about this situation.
What do you think?