Well the glib answer is that they're lost. Texts were lost all the time in antiquity, we have an utterly infinitesimal fragment of the total literary output of the ancient world. We have reason to believe that what we've got is fairly representative of the best stuff, but there are lots of gaps and authors' works are lost all the time. Texts had to be hand-copied in antiquity and the Middle Ages, a time-consuming and labor-intensive process that, though it could be done almost on an industrial scale (Atticus employed gangs of slaves whose only job was to endlessly copy manuscripts), was not always worth the trouble. Even a few decades of lost popularity could result in the near-total loss of even a great author--around the time of the Antonines, Catullus had a couple decades of unpopularity and as a result was nearly completely lost to late antique writers, some of whom lamented the difficulty of trying to find his poems. In other cases texts were lost because they were not used as school texts the way Virgil, Caesar, or Homer were--Sappho, whose lyric poetry and Aeolic dialect had long disappeared from normal Greek literature and speech, was never used as a school text and is now known only through fragments and papyri, though she was uniformly praised by ancient commentators.

Aristotle's case is a bit interesting. We have catalogs of his published works, and so we know that Aristotle wrote an enormous number of works, he was among the most prolific writers of antiquity by far. The man probably never fucking slept. Aristotle wrote two basic kinds of works, philosophical treatises intended to teach his students (as well as texts like the constitutions, a sort of empirical study or catalog), and a couple dozen so-called "exoteric" texts, intended for publication and circulation to a larger audience. These are usually called the dialogues now, since it seems that that was the structure of most or all of them. Different kinds of texts experienced different levels of popularity over time, as one might expect in such a prolific author. In the Hellenistic and Roman Periods Aristotle's exoteric texts were widely read, whereas his didactic texts appear to have been mostly ignored except by "serious" philosophers and scholars. In the first century B.C. the commentator Andronicus compiled a critical edition of Aristotle's didactic works, and from then on serious attention was turned towards the didactic works, as examples of Aristotle's more "pure" philosophical form. Already by probably the fourth or fifth century Aristotle's exoteric works had been lost--the Scholastics (in the medieval west) and the medieval commentators on Aristotle (in the east and west) seem not to know about Aristotle's exoteric works, so it seems that it's not as though medieval scribes selected only the didactic works to be preserved

Is it possible we might discover them? Sure, but it's not likely. The vast majority of our ancient texts are known from the Middle Ages, and papyrus finds are relatively unimportant in putting together complete texts, since they're fragmentary. If the exoteric works were lost already by the early Middle Ages there's very little chance we'd find more than fragments of them