Embedded in the question is this comment (emphasis added):
Compare this to something like the Acts of the Apostles, where "Luke"
claims that he is a companion of Paul who accompanied him on a few of
his missionary journeys; and yet historians are divided on whether
this is true. What makes historians so confident, in contrast, that
Tacitus did and saw the things he claimed he did?
The fact of the matter is that not all historians are so confident that all of what we have attributed to Tacitus is really authentic history, especially the Annals. For example, see the 19th century work John Wilson Ross' Tacitus and Bracciolini. The Annals Forged in the XVth Century.
However, academic consensus is difficult to reach in any era. For example, at the time of the Reformation there was a division among theologians. Many Hebrew, Greek & Patristic scholars were divided on whether to agree with what Martin Luther and what the Lutherans were arguing.
Strong feelings about religious claims can result in the corruption of historical judgment. Hence, the division among historical scholars in regards to the authorship of Luke's Gospel.
Are any or all of the works, claiming to be Tacitus, works of historical fiction? If so, could they still contain reliable historical insights related to that period of time? It's a great question to ask.
There is a lot of historical fiction being produced in our era that could easily be confused with real historical biographies. One can even read those books and end up with a flawed view of history, as some novelists use "poetic license" where false information about a historical character or period has been invented for entertainment value.
The process of determining what are works of fiction, from that of what were meant to be understood as historical biographies, has do with both genre criticism and how the documents were understood by those who wrote during the first couple of hundred years following their composition.
So, were the works that we attribute to Tacitus basically fictional compositions made for entertainment value?
As was pointed out earlier, the works of Tacitus are in dispute by some scholars. For example, in the 1899 revised Reader's Handbook by E. Cobham Brewer Wikipedia it is stated:
Annals of Tacitus (The). Said to be a forgery of Poggio Bracciolini,
apostolic to eight popes (1381--1459). It is said that Cosmo de Medici
agreed to pay him 500 gold sequins (about £160) for his trouble. We
are further told that Poggio's MS. is still in the library of
Florence, and that it was published in 1460. Johannes de Spire
produced the last six books, but the work is still incomplete. In
confirmation of this tale it is added "that no writer has quoted from
the Annals before the close of the sixteenth century." The title
"Annals of Tacitus" was given to Poggio's book by Beatus Rhenanus in
1553.
However, see the article Tacitus and his manuscripts for the pushback to that forgery argument. That article, Tacitus and his manuscripts, also puts forth a pretty cogent answer to the question raised on this post. To repost the main arguments from the article would take up too much space on this post. But, in my opinion, it really would get to the heart of a better answer that others (-4 so far!) have not seen that helpful in my answer. Another good resource is the dialog that can be found here.
Getting back to the original observation about how historians are divided who wrote Luke. To fail to examine the minority viewpoint among traditional historical scholars, believing in the authenticity of Luke being the writer of the Gospel of Luke, would be an argumentum ad populum effort that avoids the specific arguments.
To begin with, there is a title attached to most of the Greek texts for the Gospel of Luke Kata Lucan. The custodians of the texts (Alexandria, etc.) understood the phrase as referring to a specific individual and not a school of thought. They also did not understand the text as being a work of historical fiction, or even embellished midrash like expansions of historical events.
In all the manuscripts of Luke's Gospel, that have survived sufficiently intact to include any title, not one of those manuscripts omits an ascription to the author being Luke. In other words, in every manuscript that has survived sufficiently intact for any title to be present, there is a title, and that title links the text to Luke being the author.
The first and perhaps biggest problem for the theory of the anonymous Gospels is this: no anonymous copies of Matthew, Mark, Luke, or John have ever been found.
Contrast that to the question of whether we can be confident that Tacitus really wrote any of the works attributed to him, namely his Histories and Annals?
If that one forgery accusation of the Annals of Tacitus can be resolved as this argument indicates, one might argue that the custodians of the works of Tacitus did not think of these texts as works of historical fiction. They also did not understand the text as being embellished midrash like expansions of historical events. They also attributed the texts to an individual by the name of Tacitus.
For the occurrence of forgeries and their acceptance in classical antiquity see Bruce Metzger's Literary Forgeries and Canonical Pseudepigrapha.
Having written the above, the question was put forth:
Pliny the Younger, who mentions that 7.33 of his own letters that he
anticipates Tacitus' Histories to be published in the future and
achieve high popularity.
In debating the authentication of Pliny's works one might raise the question of whether its fulfillment puts the authenticity of Pliny’s material into dispute, as 7.33 could be a form critical version of a vaticinium ex eventu argument.
However, that type of vaticinium ex eventu argument can easily corrupt historical judgment. For example, using it indiscriminately would necessitate the rejecting of the authenticity of John Adam's letter to Abigail Adam, dated July 3, 1776, which reads in part (emphasis added):
But the Day is past. The Second Day of July 1776, will be the most
memorable Epocha, in the History of America.
… I am apt to believe that it will be celebrated, by succeeding
Generations, as the great anniversary Festival. It ought to be
commemorated, as the Day of Deliverance by solemn Acts of Devotion to
God Almighty. It ought to be solemnized with Pomp and Parade, with
Shews, Games, Sports, Guns, Bells, Bonfires and Illuminations from one
End of this Continent to the other from this Time forward forever
more.
