Very close to where I work is this giant billboard. I think it's the biggest billboard I have ever seen. It has been empty for years, from what my friend Omonge told me. There even may have never been an ad there!
My theory is that the advertising company who owns the billboard just doesn't have a clue on how to price the rental advertising space. Or they are not willing to pay a big enough bribe to the government official who is preventing them from putting an advertisement there.