I also live in Japan, and IMO topology (mountain slopes are not ideal to camp, most of the flat areas being owned, you'll get the police called on you),
fauna (bears, poisonous snakes, big wasps/bees, monkeys),
overgrown vegetation(in the really wild areas),
the possibility of natural disasters (floods, mud/land slides, ...) explain why the the risk-wary Japanese people prefer campsites.
Campsites are also great because you can rent all the equipment you'll only need once a year, for the summer camp few days with your family as the Japanese working force does not have a lot of holidays.
Also the equipment can get expensive fast if you want to camp comfortably, and would require storing space the Japanese people do not have at home and thus would need to rent ...
Nevetheless, I think camping is getting popular and more and more people are buying equipment.
Yeah, that's along what I think as well - apart from that I think the "getting police called on you" factor is neither a real risk nor something that really stops people from doing it.
And it has indeed been a growing trend in the last couple of years, specifically with heavy imagery and inspiration from Scandinavia.
Campsites are also great because you can rent all the equipment you'll only need once a year, for the summer camp few days with your family as the Japanese working force does not have a lot of holidays.
Also the equipment can get expensive fast if you want to camp comfortably, and would require storing space the Japanese people do not have at home and thus would need to rent ...
Nevetheless, I think camping is getting popular and more and more people are buying equipment.