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It is common that we soak raisins in water, and they acquire water by endosmosis. That means that the cells are permeable to water, right? Then why don't they lose water when kept in air? Or kept in sugar without any water?
EDIT: Also, take the example of peeled potatoes. Put them in a hypertonic solution, and they lose water, but when kept there, in air, they don't. How come?
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.
They already have! Raisins are dried-out grapes, and according to a brief search are less than 20% water by weight but over 50% sugar by weight. They've lost plenty of water already. Your average grape is probably hypertonic to most solutions and thus will not readily lose more water. Additionally, for water to be lost to air, it must be vaporized via evaporation, which is slow, inefficient from something as dry as raisins, and miniscule in effect. Sun-dried raisins already went through that process.
