It's important to bear in mind that the expected situation for a nearly 3,000 year old text is "lost." Almost all of the actual physical remains from so long ago will not have survived -- papyrus might rot, clay tablets crumble, libraries destroyed in fires, including an enticing trove of charred scrolls recovered from the ruins left by the eruption of Vesuvius... even if desert conditions might preserve things, they then have to be found again in a good enough condition that fragments can be reconstructed -- so we rely very much on most texts having survived through medieval copies; even here, we deal often with copies of copies, each of which was a significant investment in (handwritten) time and (pre-industrial paper) material, so there are many chances for a break to occur in the chain. If no one bothered making new copies of the text and keeping track of them, it almost certainly didn't survive. Thus, it needs to surprise us that certain things did survive, rather than all the things that did not.

Being particularly well-known or fundamental didn't guarantee survival: the epic poet Ennius more or less founded the Latin literary tradition, and was extremely well-known through the Latin Golden Age (he was quoted extensively by authors like Cicero, who expected his audience to be entirely familiar with the context, in much the same way that someone might nowadays toss out a bit of Shakespeare and know that an educated audience gets what you're saying), so his works certainly survived until then, but they are now essentially gone, except for the quotes used so often by surviving authors. Catullus is another tremendously influential author who was known in the Middle Ages to have existed and been important (a poem by another author survives calling him more important to the great city of Verona than the extraordinarily famous Vergil was to little Mantua) but practically none of his writing was extant until an old manuscript containing many of his poems was discovered around 1300 (bear in mind that this brings us up to the time of Dante Alighieri) which was itself then lost (copies of copies, again, are our provenance), and it is certain that he had written more which, so far as we know, are lost forever. In another example, there are few more famous ancient authors than the Greek poetess Sappho, the "Tenth Muse,' who likely wrote centuries after Homer, and who was essentially universally known through classical Greece and Rome, as we can again see through quotations of her work or references to it, including one poem by the aforementioned Catullus that was very intentionally modeled on one of hers. The corpus of her works is practically gone -- a bare few complete or mostly complete poems remain, mostly from desert papyrus finds, and one inscribed on a fragment of pottery, but mostly we now have tiny bits and pieces. Even when we have complete works, they are often just a small fraction of the author's production -- we have seven of Sophocles's plays, and the titles for more than 120. Did Aristotle write a second book of Poetics, like the plot of Eco's fantastic "Name of the Rose" would require (spoiler alert, I guess?)? We have no way to know -- Aristotle said he would write it, but that doesn't mean he ever did.

There's a claim by Pietro Bembo, writing all the way back in the 16th century, that about 1% of ancient literature survived to his day. Of course, it's impossible to know the exact figure, since a great many things will have been lost that we will never know were ever written to include in the count (ancient grocery shopping lists, although they might not count as literature -- but what about idle scribbles or poets who never got famous enough for their names to be recorded? lesser known works even by famous authors? did Cicero keep a stash of mythological fanfiction in a private journal that we'll never find out about?), and so my guess is that 1% is likely to be too high and the real figure fundamentally unknowable. Essentially, then, it's amazing that we still have what we do, and there are plenty of authors we miss just as much as the Trojan cycle that we don't.