Can't you get Japanese nationality without marrying a Japanese?

Japan is a non-immigrant country, so it is not allowed to do immigration directly.

If you want to naturalize in Japan, you can marry a Japanese. Or children born after marriage in Japan, children born in Japan are Japanese, and parents can also be naturalized.

Also, after working in Japan for six years, you can apply for permanent residence (roughly equivalent to a Japanese green card), but this is still different from Japanese nationality. Of course, there is such a requirement. According to the Japanese Nationality Law, foreigners who have been married to Japanese for three years are eligible to apply for Japanese nationality or permanent residence.