Black Ops is a shooter all about shooting people, and none of the big publishers are going to make
this. The rating doesn't have anything to do with the quality of the game either (which is going downhill because of short/rushed development times, loss of most of the important members of infinity ward, different developers (Treyarch is only doing so well because of the zombie mode and being able to piggy back off of IW's work). The early CoD games were also developed for the PC first, and their console versions weren't very good until CoD4 when they started developing with the console crowd in mind.
Big publishers aren't willing to take many risks, which is partly why there are so many FPS games, and the M rated ones sell really well hence CoD's ever increasing pandering to the console crowd. I have no idea who thinks buying a ~13 year old Black Ops is a good idea) because they also are not the target market. Dunno why the "blood" would be a problem, as many games that claim to have blood in them have weird puffs of red pixels or other strange red substances that don't really resemble blood (you can easily see worse on TV). As for the swearing, probably because they are too cheap to hire good writers who don't rely on lots of swearing for their 6 hour SP campaigns that most people only play through once or twice (if at all) that is attached to the multiplayer portion (although that does still have swearing).
Once you are old enough to have money to buy your own games and not worry about the ratings you will stop caring about them completely. I don't want to have a bunch of (with few exceptions) annoying kids running around mouthing off in my multiplayer games anyway. Not that M rated games (especially for the consoles it's a pity it doesn't keep the annoying children (that apparently are a major plague upon X-Box live) out of the game. Sure they are present on PC too but at least you have server admins who can mute or ban them.
Besides, if you have a xbox 360 get a kinect and get Dance Central 2

It's rated Teen, is a much better game and a hell of a lot of fun, especially with friends. There's also Batman: Arkham City. Although perhaps you should have looked at the games available for the Xbox 360 and their ratings before getting one in the first place.