My preferences are the opposite, but my mental frame is more about utility than about safety. I'm not worried about someone fishing for something I said that could be construed as commitment or admission - they can just as easily do that with e-mail[0]. For me, conversations can be extremely valuable, and I gravitate towards people and places where that's a common case. HN is one of such places - the comment threads here are conversations (half-way in form between chat and e-mail), and they often are valuable, as people often share deep insights, interesting ideas, worthwhile advice and useful facts. Because they're valuable, my instinct is that they need to be preserved, so that myself and others can find those gems again, or (re)discover them when searching for solutions, or read again to reevaluate, etc.

So now imagine such (idealized) HN threads transplanted to Discord or Slack. Same people, same topics, same insights, just unrolling in the form of a regular chat. All that value, briefly there to partake in, and then forever lost after however much time it takes for it to get pushed up a few screens worth of lines in the chat log. People don't habitually scroll back very far on a regular basis (and the UI of most chat platforms starts to rapidly break down if you try), and the lack of defined structure (bounded conversations labeled by a topic) plus weak search tools means you're unlikely to find a conversation again even if you know where and when it took place.

That, plus ephemeral nature of casual chat means not just the platform, but also some of the users expect it to quickly disappear, leading to what I consider anti-features such as the ability to unilaterally edit or unsend any message at arbitrary time in the future. It takes just one participant deciding, for whatever reason, to mass-delete their past messages, for many conversations to lose most of their value forever.

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[0] - Especially that the traditional communication style, both private and business, is overly verbose. Quite like a chat, in fact, but between characters in a theatrical play - everyone has longer lines.