Japan considers citizenship to be via decent. It doesn't matter where you are born, unless your parents are Japanese you are not either. Naturalisation is an option - about 10,000 Koreans in Japan become citizens every year; out of 600,000. Obviously, it's not an easy process.
If a child would otherwise be stateless, then they will acquire Japanese citizenship if they are born in Japan. But since Korea has a similar system, Korean children aren't stateless, they are Koreans.
