Why do you assume they are always drunk?
You don't have to be perfectly silent at night out in the middle of a field. A lot of those farmers actually live in a nearby town. The ones that don't probably go to bed pretty early.
So wouldn't the White House lawn be a better medium to draw on?
Why only mostly in that area of the earth to begin with?
Remember that if you told someone just 3k years ago we would be able to fly off of planet earth and then scale the moon and fly from New York to London in just 5-6 hours, they'd call you all the names. So why does flying "light years" seem like such a crazy task for a civilization that may be 100k years or even millions ahead of us?
What if they were scared shirtless to show themselves to our barbaric society? It would be like an old European who just scaled the Atlantic for the first time walking into a crazy Indian tribe.