We still live in the same society as others. People often adopt the cultures and ideologies of where they end up, or at least move closer to it than they were before. If reddit shifts its userbase to the right — even if the net effect is from “very very left” to “very left” — it will impact the lives of all of us that live in societies with large numbers of people using that site, as it’ll filter down into our politics. Even if we don’t interact with them.
For a long time, the “default” ideology of the internet was on the left. As internet usage has become dominated by a handful of sites owned by megacorporations, there has been a not at all subtle effort to nurture a conservative ideology on those sites. Stuff like reddit holding off on banning the Trump sub for however long or twitter refusing to implement their hate speech detection because it correlated too strongly with conservative politicians (not to mention what Musk has done there). I don’t think this is an accident.