Why don't raisins lose water when in air?
That means that the cells are permeable to water, right?


Correct.


  Then why don't they lose water when kept in air?


They do, but it occurs very, very slowly. It might take a few months (or years in some areas), but if you leave raisins exposed to air they will eventually become harder until the water reaches equilibrium with the surrounding air.

This slow process is why there are tips on how to reconstitute raisins that are too hard to make them more moist and plump after getting a bad batch or finding old ones in your kitchen.