In theory, they limit self promotion so that people actually participate in the community, and don't just use it as a link dump. The ideal is that people discuss other topics more often, and share their own work only every so often.
A lot of forums and chat servers have similar rules for the same reason.
Unfortunately, this doesn't work very well on Reddit, since the smarter advertisers and astro turfers have figured out how to manipulate the system well enough that people don't suspect they're advertising.
Add this to how certain types of low effort content get a ton of upvotes anyway, and how popular creators don't need to care since their fans will promote them anyway, and well, the end result is less "participate in the community" and more "don't be unpopular or bad at pretending to be someone else"
A lot of forums and chat servers have similar rules for the same reason.
Unfortunately, this doesn't work very well on Reddit, since the smarter advertisers and astro turfers have figured out how to manipulate the system well enough that people don't suspect they're advertising.
Add this to how certain types of low effort content get a ton of upvotes anyway, and how popular creators don't need to care since their fans will promote them anyway, and well, the end result is less "participate in the community" and more "don't be unpopular or bad at pretending to be someone else"