Why do Japanese speakers call Mount Fuji Fujisan?

Japanese speakers typically refer to the mountain as Fujisan because -san is the Japanese word for "mountain".1

It is a common misconception among foreign visitors to call it "Mount Fujiyama." This usage is considered incorrect because the yama in Fujiyama also means "mountain," effectively making the phrase "Mount Fuji mountain" in the Japanese language. While the exact origin of the name "Fuji" remains uncertain, one theory suggests it is derived from an Ainu term for "fire," which was then combined with the Japanese suffix -san.1