Foreigners occasionally DO adopt from Japan. However, it is very difficult, and the person who is most likely to be successful is someone who has been living there for a long time.
In Japanese culture, the "blood tie" is extremely important. Many Japanese people simply cannot understand the concept of adopting a child who is not biologically related and giving that person one's last name, inheritance rights, etc.
In fact, many people who cannot parent their children will put them into orphanages, but will not allow them to be adopted, especially by foreigners. They tend to assume that anyone who would adopt a child who is not biologically related must be doing so for evil purposes.
As a result, most of the children in Japanese orphanages are not available for adoption. However, an occasional child does become available.
There is a good discussion on the website of the U.S. State Department about the process of adopting from Japan. I would urge you to read it, at
Intercountry Adoption Japan.
Sharon