Why is it so common for people to spread their droppings all over public bathroom walls?

Common? No, I seriously doubt it, not in society in general. I'm thinking you live in a place with a lot of low lifes. I'm middle-aged and I've never seen that anywhere in my life. I've heard of it once and that was in a bad part of town. A normal person would never consider doing something like that. That is positively animalistic.