I think most of these were lost before any region had one large place known as "the dump", a relatively modern thing
https://en.m.wikipedia.org/wiki/Palimpsest
I came across this practice when reading the Chapter of Fathers in the Mishna:
"Elisha ben Abuyah said: He who learns when a child, to what is he compared? To ink written upon a new writing sheet. And he who learns when an old man, to what is he compared? To ink written on palimpsest. Rabbi Yose ben Judah a man of Kfar Ha-babli said: He who learns from the young, to what is he compared? To one who eats unripe grapes, and drinks wine from his vat; And he who learns from the old, to what is he compared? To one who eats ripe grapes, and drinks old wine. Rabbi [Yehuda] said: don’t look at the container but at that which is in it: there is a new container full of old wine, and an old [container] in which there is not even new [wine]."
Also that's a fascinating link you shared, thanks.
The manuscripts we have mostly exist because the people of the past (often Christian monks) cared enough about them to make a copy - by hand. (I assume Monks in places like China or India have been making copies in the same way, I just don't know for sure)
Gives a fascinating view into Roman mass commerce.