The problem with that is that you need to spend money to either host your own instance or pay someone else to do it for you. Most people are tech-illiterate and do not want to pay to host something which doesnt even reach the threshold to be a "hobby"
To me that starts to sound a lot like each subreddit could be replicated as an instance of something like Mastodon, and you're really only some kind of shared user database away from being able to make something that functions exactly like Reddit, but without any corporate control.