The reason for that is 99% of patients who get a colonoscopy do it under general anesthesia, risking their life for no reason, and staggering around in a drug addled haze for hours afterwards.
I had to call the hospital and demand to not get anesthesia. It wasn't even an option, just "You must have someone to drive you home after the anesthesia wears off."
Buncha big babies it doesn't hurt that much.