You gave some of the answers. There are many. Dirt in the coat or water in grass.
It's an easier way of life if they live in a high rise.
Some have too many yorkies to walk them outside and they don't have a dog run. Although, Dogs still need exercise & they need walks. It's not a demanding schedule. Dogs do need walks!
It is safer, especially for those living alone or if a man isn't present.
Some people use pads as a back up b/c yorkies are notorious for soiling indoors. Laying down a pad is a precaution for the inevitable.
All pads are easy. It takes less energy to change a pad than to clean up an accident. Not to mention it's more sanitary and less odor. You are very lucky that your boy is so good about using the potty. I bet you are thinking what is wrong with these people LOL.
Consider yourself very lucky. It sounds dirty. But my pads do not sit with anything on them knowingly. I don't know anyone that lives like that.