This isn't exactly a historical question. You might want to try r/archaeology to ask about the physical processes that tend to bury structures; I suspect this is one of their most common questions.

As for what we can answer here, while it's true that many of the structures that we find have been (at least partially) buried, that is not at all the same as assuming that most of the structures that were built were ever buried. Keep in mind that only a tiny fraction of ancient buildings survive, but those that do often only manage to last this long because they are protected by being buried. Buildings that aren't buried may crumble away completely, or be disassembled and have their materials reused in newer construction projects. Regardless of what natural processes cause structures and artifacts to be buried in the first place, one reason so many of the ancient things that we find are underground is that in many cases those are the only ancient things that still exist. This leads us to a kind of selection bias.

Also, ancient buildings and structures that weren't buried, but have managed to survive (the Colosseum, the Great Wall of China, the Pyramids, etc., not to mention old houses and other small structures) have remained in plain view ever since they were built. They aren't "discovered" (literally; the word comes from a Latin word meaning "uncover"), so they rarely attract big media events about them the way that the excavation of a recently unearthed Roman villa would.