Why should there be large caches hidden under sands? Has there really been anything like the Dead Sea Scrolls elsewhere? AFAIK those scrolls were there simply because the sect who wrote or collected them lived in those caves, or nearby. Why would anyone cache scrolls under sand (or even anywhere outside)? Isn't that quite unlikely, as a general rule?

Not under sand but the Villa of the Papyri is the best bet. Potentially thousands of rolls, definitely containing lost works. They just need to solve the xray scan and digital unfolding.

https://en.wikipedia.org/wiki/Villa_of_the_Papyri

The "xray scan and digital unfolding" has been solved.

Here's a short video about the process https://www.youtube.com/watch?v=GduCExxB0vw and here's the related publication: https://www.science.org/doi/10.1126/sciadv.1601247

I've had the pleasure to meet one of the authors of the study at a user meeting of the x-ray scanners they use, since I also work with such machines. It's a fascinating study!

Maybe the hope that some royal library at a suddenly-dried-up-oasis palace that was buried in the sands will someday be uncovered?

Seems a stretch, but I'd watch that movie.

Depending on the materials used you could also find older texts hidden in already known books. "erasing" and reusing pages was cheaper than making a new book from scratch. For example the only surviving copy of the Archimedes Palimpsest was found hidden in a prayer book.