(1) Du Fu is a cultured man, and his goodness needs time to precipitate.
Du Fu is a poet who believes in Confucian culture, so for him, cultivation is one of the most important things in life. At that time, people didn't know what Du Fu was like, and they didn't know Du Fu's sacred pursuit until his death, which is one of the reasons why Du Fu was honored as a poet. Confucius, the founder of Confucian culture, is also called a saint, and Du Fu is a loyal believer of Confucius, so people use holy words on Du Fu.
(2) Du Fu is a man with a sense of justice, and this advantage of Du Fu was not widely known until after his death.
Du Fu, besides loving self-cultivation, is also a man with a sense of justice, but he failed to shine in time because of the big environmental factors at that time. In fact, many of Du Fu's poems are about the country and the people, which also highlights Du Fu's patriotism and love for the people. But because the Tang Dynasty when Du Fu lived was very chaotic, no one noticed these things. Only after Du Fu's death did his poems spread slowly, and people realized that Du Fu was such a person.
(3) Du Fu's poems are very creative, and it was only after Du Fu's death that they were widely known.
The creativity of Du Fu's poems was not widely known until after his death, which is also the fundamental reason why he was honored as a poet after his death. Du Fu's poems pay special attention to creativity, but before his death, no one noticed it. It was not until his poems were spread by admirers such as Yuan Zhen that everyone realized Du Fu's goodness.