Because karma systems have systematic weaknesses that make them incapable of doing that in many situations.
Easy example: Let's say there are 10000 people who like cat pictures and 150 people who like long, in-depth articles. I'm one of the people who likes long, in-depth articles, so I make a forum for people to post and discuss long, in-depth articles. 10% of the cat-picture-lovers happen upon my forum and are delighted to see a new place that they can post cat pictures. My 149 friends and I downvote these cat pictures with all our hearts, but we simply can't outvote the 1000 cat-picture-lovers. Even worse, a cat-picture-lover can look at and upvote a hundred cat pictures in the time it takes me to read one long-form article, so the content that the forum is intended for is buried before even the 150 long-content-lovers get to see it. So the complete destruction of my long-article forum happens organically and by consensus, just like karma systems are supposed to work -- but it's not a desirable outcome!
Basically, karma systems are good at finding something that some group of people will like, but if that's all you want, subreddits are a bad idea, because the entire point of them is to focus on a particular thing and exclude other things that very likely more people would like to see.
Like I said, all the true* subreddits show that there are people who want to have more focused subreddits. The idea that focus should be impossible if a sufficient number of people who are don't have that focus stumble upon the subreddit just doesn't seem very reasonable to me.
Easy example: Let's say there are 10000 people who like cat pictures and 150 people who like long, in-depth articles. I'm one of the people who likes long, in-depth articles, so I make a forum for people to post and discuss long, in-depth articles. 10% of the cat-picture-lovers happen upon my forum and are delighted to see a new place that they can post cat pictures. My 149 friends and I downvote these cat pictures with all our hearts, but we simply can't outvote the 1000 cat-picture-lovers. Even worse, a cat-picture-lover can look at and upvote a hundred cat pictures in the time it takes me to read one long-form article, so the content that the forum is intended for is buried before even the 150 long-content-lovers get to see it. So the complete destruction of my long-article forum happens organically and by consensus, just like karma systems are supposed to work -- but it's not a desirable outcome!
Basically, karma systems are good at finding something that some group of people will like, but if that's all you want, subreddits are a bad idea, because the entire point of them is to focus on a particular thing and exclude other things that very likely more people would like to see.
Like I said, all the true* subreddits show that there are people who want to have more focused subreddits. The idea that focus should be impossible if a sufficient number of people who are don't have that focus stumble upon the subreddit just doesn't seem very reasonable to me.