Actually, I don't find it preposterous in the least.
We have a large amount of evidence that a new religion was founded in Palestine by a Jew named Jesus around 30 AD, and that that religion very quickly came to assume such importance that our very calendar derives its system of numbering years from the date of the birth of its founder.
There are literally dozens, if not hundreds, of important potential artifacts from this early period of the history of this religion that could, and by all reasonableness, should have been preserved in some way.
Possessions of Peter and Paul, and the various other apostles, and undoubtedly bones, burial sites, ossuaries and many other things relating to the early church and its leaders would have been preserved. Some would eventually have gotten destroyed, others forgotten.
Many such artifacts would have been carefully put away in secure (and in a great many cases) secret places. It is neither strange that they should have been hidden away, nor that, in many cases, what the artifact actually was should be forgotten over the centuries. A single family or group of people will have been the caretaker of a particular artifact. At various points, it will have been deemed wise to limit the knowledge of such an artifact to a few, or even to only one, person. Or perhaps an elder caretaker simply didn't get around to passing on the knowledge before meeting with an unexpected death. Over the couse of 2000 years, the chain of knowledge was broken.
We have manuscripts of New Testament writing -- far more fragile than stone -- that were penned very early on, including the essentially complete Codexes Sinaiticus and Vaticanus. It is not at all strange that a stone artifact should survive.
Nor is it strange that we should find it now, at a point in history when there are people poking their noses into just about every nook and cranny on earth, when knowledge and communication are widely available, and when intensive research is doubling the knowledge of mankind every few years.