Why can't the cattle just walk right up to the barn, you may ask? Because cattle are really damaging. They love to rub up on things like trees and buildings. I've seen them rub so hard on a barn that it wore away the foundation. I've seen them kill trees by rubbing on them and licking them and eating them until they're dead.
As a result, we don't want them to hurt our new barn because it looks so shiny and new! And like most new things, we want to keep it looking that way as long as possible.
Sure, it has manure in it, but that doesn't mean it can't look nice from the outside.