Oh but they are! When I was a child (1950) I was put in a small hospital run by Catholics. The nuns terrified me so bad for years when I saw one on the streets I would dive for the floorboards of dad's old car as I thought they might get me and take me to the hospital again. I was only 4 years old.
A few years later I decided I didn't need to be scared of them and have not been scared since.
I have noticed the upswing in horror movies using nuns as the villain.
Ah! They're doing a story about my 3rd grade Nun!