You'll notice that the entire source code for reddit is hosted publicly here (https://github.com/reddit/reddit https://github.com/reddit/reddit) which makes you

You'll notice that the entire source code for reddit is hosted publicly here (https://github.com/reddit/reddit https://github.com/reddit/reddit) which makes your question that much more interesting. And the answer? It's the community and the centralization. By making a subreddit, you have a chance at scoring some of the people who already visit the site, rather than having to look for them or advertise on your own. Discoverability aids the community, but it's only made possible by centralization. Then there are things like global shadowbans and antispam and the fact that karma is shared across reddit.com. If you made a separate reddit, karma on your site would not be reflected to the original. Which would suck. Reddit.com dominated the reddit market for just about everything.