Why does writing clouds refer to poetry and songs?

Because clouds come from ancient times. Clouds, referring to smoke and clouds, describe illusory and misty things or things. The smoke cloud comes from Du Fu's Song of Drinking Eight Immortals in the Tang Dynasty: "Zhang Xu has three cups of grass, bared his hat in front of the maharaja, and waved paper like a cloud. Tang Li Bai's "Antique" 46: "The princes are like stars and moons, and the guests are like clouds. Waiting for the poet's poem.