Because advertisers have bought into the common and mostly true attitude that humans do not have the mental capacity to comprehend subtlety, and accordingly have resorted to Shaiya and Evony-style ad banners. (Speaking of which, Evony keeps getting worse and worse with each iteration of their stupidity.)
I don't get it. How is this monkey advertisement subtle in any way? If anything, I would feel stupid for buying a monkey for $18.95 after seeing this.