Japanese food is considerably less spicy and 'hot' than the food that most Koreans and Chinese eat. Obviously there are a number of dishes that are spicy in Japanese cuisine, but as a whole and compared to other East Asian cuisines there are only a few dishes that are spicy. Why is that so?
Korean or Chinese food is not necessarily very spicy. This may be the Western impression given the prevalence of Korean kimchi and Chinese Szechuan cooking (both very spicy) in the West, but Korean cuisine is far more than just kimchi whereas Szechuan is just one of the regions of China.
Modern Korean cuisine in fact emphasizes balance in all the different flavors; while Chinese is so diverse that food widely varies from one region to another.