It’s more like judging all of 4chan based on /b/. And the thing is, even if you go into the unrelated parts, it’s still just as bad - the tabletop communities around 4chan are still chock full of white supremacists, fascists, homophobia, and all sorts of bigotry, to the point where I know just never to game with people from them.
Redditors don’t just stay in their own neat little corner, and the entire website is dripping in awful. Just look at how we already have people in this thread who insist that their own subreddits are perfectly fine, when they’re outright dedicated to hatred, harassment, and bigotry.
That’s not to say that all subreddits are shitty, you’ve just got a really high chance of a subreddit being absolute crap, because the entire website is infested with assholes, and has cultivated that culture.