They are, but that's one of the reasons you might have to wash them. If a dog gets covered with something noxious, it's generally not going to ingest much of it. At cat will eat everything that's on it's fur.
The other reason I've washed a cat was on a day of 40 and the cats were suffering. A soaking lowered their core temperature, and then they licked the water off, making sure they were properly rehydrated.