Seems we discover ancient greek writings all the time(though most are incredibly fragmentary and are rarely translated) in the Egyptian desert. Though for latin that appears to be very rare that new Latin works are discovered in this manner.
That partly depends on what you mean by discovery. You're right that archaeological discoveries of previously unknown Latin texts aren't very common, but they do happen. The Oxyrhynchus Project, for instance, has uncovered thousands of Greek and Latin texts on papyri in trash heaps at an Egyptian site. At Qasr Ibrim, also in Egypt, a scrap of Cornelius Gallus's poetry was discovered in 1979. Just like the Greek finds you mention, these are all fragmentary, and the Oxyrhynchus finds are documents from everyday life rather than seminal works of classical literature.
However, many discoveries of Latin texts are actually rediscoveries of things which have been in private hands or have been sitting in archives, untouched for centuries. This page offers an incomplete list of some of those sorts of finds. Unfortunately, many archives don't have complete catalogs of their holdings. Even when they do, they may not include information about fragments of texts contained within a larger manuscript. Check out these pastedowns, for instance: one, two, three. These are fragments of old manuscripts that scribes pasted into the bindings of new books to hide the seams. Pastedowns are generally in pretty bad shape, but they do sometimes offer new fragments that we didn't have before. Check out this post from the Hill Museum and Monastic Library on projects in process to work with these kinds of materials.
The problem with "discovering" Latin texts is that we already have an awful lot of them -- so many so that we're really not sure what we do have, which is one reason archival research is so important! On the one hand, we're fortunate in that medieval and early modern scribes and scholars preserved a huge amount of material for us in Latin, more so than in Greek. The circulation of manuscripts in the Middle Ages means that we've got multiple copies of many texts. On the other hand, some of the places where that material was collected were destroyed in the world wars, so there may be things which are entirely undiscoverable.
*Edited to add that although I didn't see the other comment before it was deleted, the text found in a bog is almost certainly the Faddan More Psalter, found in Ireland in 2006. While it's a fantastic find and hugely important as an artifact, it's not actually a new text -- it's from the Vulgate.
