Du Fu, a poet in Tang Dynasty, was called "Poet Saint". Why is Du Mu called by later generations?

Du Mu doesn't have a poem or anything.

Du Mu, a native of Mu Zhi, is called Du Weizi. (Source: Zhongshu Province is another name, so people call it "Du")

Du Mu is called "Xiao Du" and Li Shangyin is also called "Du Xiaoli". Because he lived in South Fan Chuan Villa in Chang 'an in his later years, he was later called "Du Fanchuan" and wrote "Collected Works of Fan Chuan".

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