Why are Du Fu's poems called epics?

The theory of "the history of poetry" was first seen in Bai Meng's "The Art of Poetry" in the Tang Dynasty: "Du Fu (Fu) met the difficulties of Lushan Mountain, moved to Gansu, Sichuan and wrote poetry, and almost disappeared, so it was named" The History of Poetry "at that time. "Du Fu was a great realistic poet in the Tang Dynasty. He wrote his own experiences and feelings in the Anshi Rebellion with poetic pen, which comprehensively and profoundly reflected the social reality of this period, so he was already called" the history of poetry "at that time. For example, the famous narrative poems "Three Officials" (Xin 'an Official, Shi Haoguan Official, Tongguan Official) and "Three Farewells" (Newly Married, Farewell the Old, Don't Be Homeless) are full of deep sympathy, revealing the profound disasters brought to the people by this war and showing deep feelings of worrying about the country and the people.