Most people spend about half an hour in the bath every night. Most children take their baths with their father or mother until they are in the upper grades of elementary school. The family tub is an important place for parent-child communication. One answer might be the climate. Japan's summers are hot and humid, and its witeres are cold.
Hot baths wash the body's sweat off in the summer and warm the body up in the winter.
Another answer might be Japan's volcanoes. Japan is famous for its number of volcanoes, many of which are still active. In many places, hot water bubbles up from underground. These hot springs, and the presence of many fast flowing rivers with clean water, have influenced the development of Japan's bath culture. Japanese use their baths not only to get clean but to maintain their health by warming themselves up and stimulating their circulation.