IIRC that's more a phenomenon with clay tablets than papyri – and it's an anomaly because of a rather unique combination of extremely sturdy media (clay tablets

IIRC that's more a phenomenon with clay tablets than papyri – and it's an anomaly because of a rather unique combination of extremely sturdy media (clay tablets especially) and extremely well-preserving environmental conditions (dry deserts). And without the Rosetta stone, they'd be incomprehensible still. Preserving texts and preserving knowledge of a language are two separate problems.