I took down a couple of pieces of beat-up sheetrock and found that many
of the cavities were almost completely full of pieces of sheetrock.

Any good reason why a builder (or remodeler, perhaps) would have done
this? The only thing I can think of is that it saved them the trouble of
carting the offcuts away and perhaps paying to dump them.

I suppose they haven't been doing any harm, but they are a pest if I
want to install an old-work outlet box in a section of wall where I do
not remove the sheetrock.

Perce