And at what frequency would it save the payload? In 85-95% of cases, payload is safely delivered to space, so the dummy just slows things down. In most of the remaining cases the payload is lost on it's actual ride into space (e.g. the last time SpaceX had a problem, CRS-7) where a dummy can't help. There have been a handful of pad accidents involving loss of payload, but they are very much the exception to the rule about the exception to the rule.