The biggest problem is that many of the best known and most important texts describing the history of Rome cannot be reliably traced back further than the 13-15th centuries, and there are strong arguments that some of them were forgeries (which in turn, casts doubt upon the rest).

The fact that many historical documents are forgeries or were edited for political reasons is not in doubt. The Donation of Constantine is one notorious example, in which a document that claimed to be "a forged Roman imperial decree by which the emperor Constantine the Great supposedly transferred authority over Rome and the western part of the Roman Empire to the Pope" (quoting wikipedia). In fact it was forged some time in the 700s.

During the Renaissance there was a craze for collecting works of antiquity, to the extent that the Catholic church actually posted large rewards for the discovery of previously unknown manuscripts. Not surprisingly, large numbers of works that were previously entirely unknown or merely rumoured were "discovered", often in circumstances that to a skeptical modern eye seem completely implausible.

A typical origin story involves a scholar/adventurer type venturing to a remote monastery, upon which a previously forgotten ancient manuscript was located. The monks that toiled at the abbey copying these manuscripts throughout the dark ages were invariably ignorant of the great importance of what they had, until the discoverer presented the manuscripts to an amazed and delighted world (and collected a reward).

Examples of texts with such stories are the Annals of Tacitus, the Poems of Catellus, and the History of Velleius Paterculus.

These stories have repetitive, severe problems that would be torn apart in a world that had snopes.com, but of course the medieval era did not. In no particular order:

• The texts frequently have a single source document, with none others ever being found. That document was invariably treated with little caution, meaning that the oldest copies of the text are said to be copies of this one original manuscript. It's implausible that works that were recognised immediately upon "discovery" as great works of literature had literally a single copy in the entire world.

• The origin stories are sometimes absurd, like the Poems which is claimed to be discovered under a beer barrel.

• The texts are often not cited at all or their existence is barely even mentioned from the time they were supposedly written up until the time of their "discovery", at which point citations suddenly appear all over the place. It is difficult to believe that such large and historically important works could be barely noticed by scholars for literally centuries.

• They sometimes have mistakes that a contemporary author could not possibly have made, e.g. in Tacitus' Annals he refers to London as "remarkably celebrated for the multiplicity of its
merchants and its commodities" although at the time the book was supposedly written London was little more than a village around the size of Hyde Park at the far end of the empire, and wouldn't have been known to anyone in Rome. A medieval forger with little knowledge of actual ancient history, on the other hand, could have easily made this mistake.

The more you dig into the back stories of important ancient documents, the more you discover that if they had simply been invented for profit, we'd have absolutely no reliable way to discover it.