Most powdered detergents be they for laundry or automatic dishwashers are quite similar. All contain washing soda, sodium silicate, phosphates and other powdered chemicals which will over time absorb moisture. This is why their containers are labeled "store in a cool and dry place", and "keep lid (tightly) closed.
On another note as powdered detergent ages it looses ability to disslove well. The sludge that remains can gunk up a dishwshwer's pump and other parts, especially in hard water areas.
The first attempts at automatic dishwasher "liquids" was pitiful, but if the bottle of Cascade "Complete All-In-One" gel is any indication, things have improved. Methinks as with laundry detergents, the day will soon come when liquids, gels, and liquid/gel "packs" will dominate the shelves where powdered automatic dishwasher detergents once held sway.
Tablets, OTHO have a limited shelf life before they turn into bricks. This occurs regardless of individual wrapping and being inside a box or plastic bucket with a tight fitting lid.
L.