... . They are subject to oxidation and must be sealed - but the same is true for all photo elements. Being quite big, selenium cells (and silicon cells) are harder to seal than photodiodes/phototransistors, and being older, the sealing materials used on selenium cells often are inferior to those we use on modern photo elements. And if the seals fail, humidity and a current across the cell (the latter created by exposure to light) will speed oxidation.
There are cell makes that held up extremely well (the big round cells used in Norwood, Weston and Sekonic style pro meters frequently are still fine), and other types that rarely survived. . .