No community I've ever seen, apart from MetaFilter, has kept the noise level low enough that participating in the community is enjoyable. Hacker News was great a few years back, but it's painful to watch now. Same for every other site I've known: As the community grows, discussions degrade, and people get hostile.
People don't have a lot of insight. So when sites open up to comments, the typical comment — which may be perfectly average for humanity — is trash. People have agendas and biases. Most people don't see it, or don't care, so they don't correct for them.
Commenting systems and algorithms can hold back the tide for a while. They can force out people with a net negative impact on the community. But in the end, attempting to sustain good discussion is a losing battle. It's so easy for good discussions to turn bad, for intelligent arguments to become baseless, for disagreements to become personal attacks. Humanity as a whole isn't capable of rational, reasoned discussion at scale.