Why does the official history of a dynasty have to be revised by the dynasty after it? Why don't you revise it yourself?

Every monarch in each dynasty will have official recorded history, such as daily records.

Later dynasties basically used these as reference materials for compiling history. No dynasty would compile its own history books. The tradition of the Twenty-Four Histories of China is: after the change of dynasties, the history is compiled to evaluate the entire dynasty. When the previous dynasty was not destroyed, it was the compilation of history.

Even if it is done, it will not be recognized by later dynasties.

For example, the records of Li Shimin's brother-killing in the Old and New Tang Books are mostly considered untrustworthy by later generations, but why didn't the Song people revise their history and change it?

The reason is that Li Shimin has tampered with all the records of the Xuanwumen incident in the current dynasty. Later generations can only compile history according to these historical books. Even if there are doubts, they must not violate the historical books of the previous dynasty.

Besides, there are some things that cannot be known to the subjects, so later dynasties must deliberately conceal them.