Why do people praise Du Fu's poems as a "history of poetry" and Du Fu as a "poet saint"?

Du Fu only worked as a small official intermittently for a few years in his life, and his political career was quite short. However, as a great realistic poet, he always cared about politics, society and people. His initial ideal was to "respect the monarch Yao and Shun, and then change the customs". Later, he wrote in the poem "From Beijing to Fengxian, chanting 500 words": "I am worried about Li Yuan in poor years and sigh for the heat in my intestines." Until his later years, he still didn't change his mind. "Sleepless and unable to work hard" is a true portrayal of this mood. Although his political ideal can't be realized, it is actually reflected in his poems. It is precisely because of this that his poetry is great, and he is known as "the history of poetry" and "the sage of poetry".