Lots of old books survive. They survive because someone cared enough about it to make a copy. Before the printing press, that was entirely by hand. one of the main purposes of those monasteries you mentioned is to copy relevant works for the next generation. Well, maybe I should say the next ten generations or however long books last depending on the local climate.

Burrowing stuff sometimes works, however, a lot is lost. Even when we attempt to save things, it often doesn't work. Some years ago a town near me had a big unburying of a vault they had saved for 50 years. In there was a brand new car complete with gas in case gasoline was obsolete in 2000. The gas was stale and unusable. The car was completely rusted the point where it couldn't be restored. The only answer that you can be confident of storing something is if someone every few years personally looks at it and verifies it is still in good shape and if not repairs it. Archaeologists can learn a lot from garbage and trash that was buried with no attempt to save it. However, archaeologists are also very aware of the limitations of what they can get. There is a lot they know they don't know because, well, this would not have been saved or been preserved.