For the same reason you would put a safety on a enclosed hammer (hammerless) gun - to mitigate unintended hammer fall. Obviously, an exposed hammer gun is safe from unintended hammer fall when the hammers are cocked only to the halfcock notch; barring a blow serious enough to break the trigger nose/halfcock notch. The position of exposed hammers, cocked or not, is generally obvious to anyone interested. The fly in the exposed hammer gun ointment is having to cock them just prior to firing. The equal fly in the "hammerless" gun ointment is that the unexposed hammers are generally cocked and can't be seen. Therefore, a safety is more or less required equipment on hammerless guns. A safety is more or less redundant on an exposed hammer gun unless one wishes to carry it cocked --- as one does with a hammerless gun.
