Even many foods that do not require Hashgacha have a Hashgacha. At times, after a book, CD, or DVD hits the market, there is a proclamation banning it. Perhaps, instead, a book, CD, or DVD could have a Hashgacha certifying that it is Kosher for the soul. Similar to the case of food, there can be different Hashgachas with different standards, and everyone can follow the standard they deem proper. Why has this not been done?
In order to give a Hashgacha for an item, you need to be able to determine if it is indeed Kosher or not Kosher. There must be clearly defined rules. For food, these clear rules exist in Halacha.
However, for media and literature, there are no such rules - it's all a matter of opinion. In other words, it's not Halacha, it's Hashkafa. Some would say that a book is Assur, others would say that it's not Assur, but not a good idea to read, and yet others, Muttar entirely, and in fact, valuable lessons can be learned from it. (I refer you to the controversy concerning the Harry Potter books.)
You mentioned that there should be different standards. In practice, this would just lead to mass confusion, and probably fights whether people are "frum" enough. Since it is so subjective anyways, why don't we just let people make their own decisions, based on the Hashkafa they've learned from their rabbis and community?
Finally, there are many more books than there are foods, not to mention music and films. It is simply not practical to review and certify every item.
It is possible that certain tight\isolated communities could start certifying certain books as kosher for their community, but these communities tend to prohibit all outside literature to begin with...