Outside seating is relatively not common in Japan. Although Japanese like picnic and BBQ, most of the restaurants and cafes do not have outside seating areas. Therefore, Japanese like to sit outside when they bring their own meals or cook by themselves but they have to stay inside the building when they dine at restaurants.
In addition to behavior, the law do not allow restaurants or cafes to place their chairs and tables on public spaces - sidewalks. Anyway, the sidewalks are not wide enough to accommodate these chairs and tables.
Then, may be the weather and climate are not suitable for sitting outside. Precipitation in Japan is much higher than in Europe. The average precipitation of Tokyo is 1528.8 mm/year but Paris is 619 mm/year. Rainfalls in Tokyo during winter is low but it is too cold to sit outside.
Popular outside seating is during summer when temporary beer gardens open on roof tops. These beer gardens do not have inside seating area, therefore there is no place to escape when sudden rainfall begins. Therefore weather forecast is important for Japanese, especially for restaurants.