Evidence for this common myth is lacking.

https://www.thearchaeologist.org/blog/the-great-library-of-a...

For some reason, there is a gigantic and ancient monastery on Mount Sinai with a commensurate collection of ancient manuscripts and papyri. Totally coincidence.

https://en.wikipedia.org/wiki/Saint_Catherine%27s_Monastery#...

How did all that stuff get up there? It was holy angels. #itsalwaysangels

This is a common refrain but in reality I'm not sure it made much difference. Papyrus just doesn't age well and most manuscripts from this era would've been on papyrus.

What really decided what texts survived and what didn't was monastic traditions in in the Dark Ages and Middle Ages [1]. At this time, a monk might spend their entire life transcribing a particularly long manuscript. The materials were also expensive. So monasteries were selective in what got retain and unsurprisingly it skewed heavily to texts of religious significance and then to texts of significance to, say, Roman and Greek tradition and history given that monasteries were European.

[1]: https://spokenpast.com/articles/medieval-monks-erased-preser...

Everybody knows it's under Uncle Scrooge's money bin. Spoiler alert.