HN has millions of users, so of course there's a wide spectrum of views. Any view that is sufficiently represented in society as a whole will appear here, regardless of how bad the argument is or feels. Moreover, HN users are all over the world, and geography (as a proxy for background and culture) has a huge influence on how people view these things. Surprisingly many of these disputes on HN come from people in one part of the world arguing with people in another part of the world without necessarily realizing it.
So while you're right that someone regularly "comes along to say $X", that's a function of population size and distribution. You can't generalize from that to the community as a whole, any more than to the human race as a whole. In the latter sense, sure, maybe "we" should really be above $badness by now, but saying that doesn't contribute to the hard work of persuasion, and neither does condemnation or name-calling or fulmination—however justified those reactions feel, they just contribute to cementing things as they are.
Edit: just for clarity:
I understand and sympathize with the frustration when you (I don't mean you personally) feel like a site is 'your' community on the internet and then you run into posts that aren't just wrong but are squarely outside the bounds of what decent people believe or say. It creates a feeling of a hostile environment that is even worse than if you had gone in expecting hostility. I wrote about this here: https://news.ycombinator.com/item?id=23308098.
The problem though is that those bounds are drawn very differently in different subgroups of society (and the world at large) and the population here is large and distributed enough to include all those subgroups. That means it's constantly and unavoidably pregnant with conflict. For most of us, any social conflicts we're implicitly part of are insulated and regulated by the subgroup(s) we belong to. In our world, for practical emotional purposes, approximately 'everyone' agrees. There may be an occasional holiday family dinner where the larger divisions break out, but (at least for a lot of us) that's about it. The population as a whole, however, can't regulate itself that way. A population as large as HN is in the 'hostile family dinner' situation all the time, only orders of magnitude more complex and worse, and spread all over the world.
I don't think the solution can be to exclude subgroups. It has to be to communicate across differences. One could call it 'supported communication across differences', where by 'supported' I mean having a scaffolding of principles that everyone is expected to follow (https://news.ycombinator.com/newsguidelines.html, in HN's case.)
In practice, yes, that does mean going backward and having arguments that "we" should be beyond by now. But that's because there are many different versions of "we" here. If you take all the basis vectors of HN and consider the full opinion space they expand into, I'm not sure it's that much less complex or diverse than the entire world.