Common sense is not common. Why do you think that people have to put on warning labels on obviously dangerous stuff. "Rat poison? Doesn't say it's dangerous, so it must be okay to drink!" "Oh, look! An open fuse box! Let's poke it! " People are generally dumb. Very, very dumb. The trouble is, the dumb ones don't realize how dumb they are, and often form groups of dumb people with a collective intelligence lower than that of any individual. I call it the "hold my beer and watch this!" effect.