Maybe it pops up here because people have at least a modicum of hope that there will be a productive conversation even amongst the various downvote brigades?
I post political comments because even when they get downvoted to -4, they still end up with a long list of replies and sub-tangents in response to them. I think that's a healthy thing.
What people want to do and what should be done are entirely different things.
There's an interesting dynamic to this, btw. If HN manages to stay a degree or two more interesting than internet median [1], it attracts high quality users. That makes it a desirable audience. That makes a lot of people want to target this audience, so they blast it with rhetoric. Rhetoric isn't curious conversation and it thrives on repetition—so it makes HN worse.
In other words, to the degree that HN gets better, it gets worse. There's a cap on how good it can ever get [2].
[1] I'm not saying it's very good at this. But it's all relative, and what matters is outrunning the bear: https://news.ycombinator.com/item?id=25725436.
There are a few other places you can do this, like the slate star codex culture war comment spinoffs: r/theMotte and https://www.datasecretslox.com/index.php
Due to rightwingers often feeling unwelcome elsewhere a lot go to those places, meaning both communities think they need more leftists to balance things out.
Frankly, their election fraud thread - which has hundreds of comments demonstrating both extreme misunderstanding of statistics and the voting process, coupled with a willingness to make or support incredible claims without any substantive evidence or knowledge of the subject matter - says a lot about that community, in my view.
Instead, the decay of social media happens as the platform transitions from a place where one talks with people to a place where one talks at people.
If I reply to your political thoughts by telling you off, I'm probably not trying to convince you to change your mind. Instead, I'm performing for the attention (and upvotes / retweets / kind comments) of like-minded peers.
That's obviously alienating for the person who gets attacked, but this kind of performance is also self-radicalizing. The validating reinforcement preferentially goes to the strongest attacks or defenses, favouring rhetoric (as you noted) rather than substance.
One of the few things that suppresses this cycle is exactly what HN does reasonably well: have the community be about something else, diluting political content such that there's less often a chain-reaction.