Part of the problem is water composition. Water which contians fertilizers, surfactants etc is going to damage the protective waxy layer on leaves, while rain water will do so less. Next, when watering youre doing so as frequently as the plants can make use of, to encourage maximal growth rates. It would never rain that frequently. finally, the size of droplets is different. Rain drops tend to be big and fat and roll off the waxy leaves while smaller dropplets sit on / stick to the surface where they create a nice wet and likely to rot environment.