Why are sponges damp when you open the package?

Sponges are moist because manufacturers wet them just before packaging. Typical cellulose kitchen sponges get hard, wrinkled, and crusty-looking when dry. But adding about half of an ounce of water restores them to new heights of consumer-friendly glorious sponginess.

The only reason that sponge packages are airtight is to keep the moisture in, and the water used is purified and distilled. This is done to make sure that there are no bacteria sealed in with the sponge. Otherwise, the moisture would be a potential breeding ground for all sorts of nasty stuff, and the sponge would be spreading germs instead of cleaning them up.