The first reason is that the Romance of the Three Kingdoms is wrong in many places compared with the real history. Reading the Romance of the Three Kingdoms as history will make us receive the wrong historical information. After all, the romance of the Three Kingdoms is just a kind of? Romantic? Literary works belong to fiction. Although many plots are based on history, some additions and deletions have been made to the official history. If we regard the contents of The Romance of Three Kingdoms as official history, we will misunderstand and even distort history. If you want to know the real history, you still have to read history books. For novels like The Romance of the Three Kingdoms, we can just joke and don't have to take them all seriously.
The second reason is that The Romance of the Three Kingdoms was not written by historians, but by the author of the novel, which added many subjective factors to the plot. For real history books, the editor has no subjective factors to record, and the judgment of people and things is left to future generations to complete. Romance of the Three Kingdoms is different. As a novel, it needs decent and villains, and it needs twists and turns to attract readers better. The Romance of the Three Kingdoms just took this route, setting Liu Bei Group as a positive task, Cao Wei Group as a villain, and Sun Wu Group as a soy sauce player, which is too subjective.
The third reason is that The Romance of the Three Kingdoms only restored the historical situation at that time to a certain extent, but there were also many historical plots that were only brushed aside or even left unsaid. Novels have different emphases, which we can understand. But history does not have its own emphasis, and it needs to record all the real information. If the Romance of the Three Kingdoms is regarded as history, we may not know a lot of historical information.