Because of the word 'Romance' and its largely different accounts on individuals actions during that period. Also, the Mao family's change made it even more romancing.
Bearing all that in mind, who is to say that Luo Guanzhong's only reference was the Records of the Three Kingdoms and that everything else was his imagination?"
Making them historical fiction.
People make real persons into heroes by embellishing the events not inventing events.
Like what? Did Anthony really run almost naked across Rome and let people touch his penis? Did someone really walk up to Caesar and tell him, beware of Idles of March? Did Caesar really say Et Tu, Brutus? We know Anthony DID NOT make that speech at Caesar's funeral, etc.
Shakespeare wrote about Julius Caesar 1600+ years after his death but the play is fairly accurate, so if people performed operas who is to say those operas are not based on reality?
Fact is, that period of history have been much obscure by Octavius' correction on history, so we honestly don't even know our sources are correct, let alone some romancing of Shakespeare's retell.
Actually, no proof of something means there is no proof. Anything could of happen, including aliens or divine ones making them happen, and its as likely as your scenarios, thus your scenarios is pretty much invalid as the probability of it occur is 1/a very large number.
People often say that there is no proof that certain events happened so they most not have happened. That is an illogical statement. If there is no proof that an event happened that means nothing if there is no proof that it didn't happen.
Again, folk tale are often use to damage people's reputation. To say just because the common peasants of that time believe something making them somewhat true is pretty stupid.
The fact that a folktale exists regarding the event is in fact closer to actual evidence then some academic saying it didn't happen because he or she doesn't think that it did, because folktales are often based on truths. If you look at the folktales of Vlad Tepes, you understand why Stoker chose him as the reference for his vampire. Was Vlad undead? No. Was Vlad responsible for the murders of thousands of people by impaling? Yes. Did Vlad drink blood? Maybe, but there is no evidence that he didn't, and some evidence that he did (wooden carvings for printing blocks for German pamphlets exist that warn people that he did drink blood). In this case the folktales are very close to the truth. It is only because other evidence exists that we know these events actually happened, but the folktales agree with a lot of what actually happened. So the operas and folktales would have at least some truth to them and therefore a fair amount of RTK could easily be true.
They say a baroness bath in blood of hundreds of virgins, and peasants swore them. Does it make it true? Probably not.