The MPAA is a trade association, it’s not really so much that they are “worried” about your daughter, it’s what they fear will offend you. In general the MPAA seems to have decided that parents will be more offended if their kid sees boobs in a movie than violence, so movies with boobs get an R rating so unwitting parents won’t complain. The MPAA was probably happy not even doing a rating system, it started doing it because of people complaining about sex and gratuitous violence.
Too much violence can/will get an R rating though, in general graphic violence is always R-rated.