It has yet to be proven that intelligence has survival value.
--Arthur C. Clarke

The problem is that for most cases, stupidity isn't painful. Making a mistake at work, your co-workers can cover for you. All too often there are nets of some sort or another that catch us as we fall from stupidity. Finger pointing is easy, you can always find a stupider person. Homer Simpson is a hero for too many people.

Other times it may be stupid, but it realized something. Many accidents in the world have lead to important discoveries. Where would we be without penicillin, a most fortuitous accident?

For other images...
Just think of the stupid masochist, who would go about walking with his shoes untied so that he can get a constant buzz of pain. Don't even start on the various forms of drug induced stupidity.

Do you really want the sadistic job of zapping most of the population of the world?

Stupidity cannot be cured with money, or through education, or by legislation. Stupidity is not a sin, the victim can't help being stupid. But stupidity is the only universeal capital crime; the sentence is death, there is no appeal, and execution is carried out automatically and without pity.
--Lazarus Long