Because those who rate movies are trying to prevent kids from being exposed to too much violence, language, sexuality. Just because a child swears doesn't mean that its acceptable for them to be exposed to more.
Children are not ready to deal with alot of the adult sitations in R rated movies. In fact, I saw "White Noise 2" the other night, which is rated PG13, and frankly, if I had kids, I wouldn't let MY 13 year old see that movie. WAY TOO MUCH violence. Its not acceptable. It gives kids the wrong idea about life. Kids are growing up way too fast these days, and its mostly due to the adult situations in the media and in movies.
And don't think that I'm some 50 or 60 year old who can barely relate to being a child. I'm only 29, it wasn't that long ago that I was in your position, but, trust me, once you get into your 20s you'll see what I mean.