In many Japanese bathrooms, the shower and washing area are located outside the bathtub. That layout reflects the usual order of bathing: wash first, then get into the tub.
In a Japanese home, the bathtub is mainly a place to soak, warm up, and relax. Hair and body are washed outside the tub, and soap and shampoo are rinsed off before getting into the bathwater.
One important reason is that family members often use the same bathwater one after another.
The tub is not usually drained and refilled after every person. Because the next family member may soak in the same water, it makes sense to wash off sweat and dirt beforehand and keep the bathwater as clean as possible.
Even within a family, people generally expect the person before them to wash properly before entering the tub. If someone got in without washing, it might not mean the water had to be thrown away immediately, but it would still feel unpleasant. Cleaning the body before entering the bath is the basic expectation, even at home.
At public baths and hot springs, another common practice is kakeyu. Before entering the bath, people pour warm water over their bodies, often using a small hand bucket. This helps rinse away sweat and surface dirt, and it also lets the body adjust gradually to the temperature of the bath.
Kakeyu is not the same as fully washing the body with soap. Its importance is that you do not simply step straight into a shared bath without doing anything first. At a public bath or hot spring, where many unrelated people use the same water, keeping the bath as clean as possible becomes even more important.
So Japanese people do not wash first simply because they are unusually concerned with cleanliness.
The bathtub itself has a different role: it is not mainly where the body is washed, but where a clean body soaks and warms up. Because the water is shared with other people, washing before soaking and practices such as kakeyu naturally became part of the normal bathing routine.
