Presumably this is because of the social convention where you’re expected to back up any public belief with arguments, so that other people can attempt to poke holes in them. I find this strange because the arguments people present rarely have anything to do with why they believe those things.
We post our beliefs hoping to convince other people. Even if we're not certain of a belief, we generally intend for the readers to approach our own degree of confidence.
We give the best arguments we know for the beliefs (and if we're good rationalists and are also honest and confident, we also post the best arguments we know against them.) Sometimes these aren't the true reasons we hold those beliefs, because we're not luminous or introspective enough. But they are still arguments that can be evaluated and they are legitimate, admissible evidence that people should indeed update on.
If we didn't give any evidence but just stated our beliefs, then the only reason anyone would be convinced by them would be that they trusted our strength as rationalists. And such trust is generally misplaced, precisely because as you point out, even when we try to honestly analyze why we believe something, we are often mistaken, biased, etc.
If you also consider dishonest, manipulative, or just plainly irrational actors, then the mere fact they claim to believe something is very hard to update on correctly. Whereas the justifications they give for the belief can still be evaluated and provide at least some evidence.
I like posts that are concise and to the point. Posts like that maximize my information/effort ratio. I would really like to see experienced rationalists simply post a list of things they believe on any given subject with a short explanation for why they believe each of those things. Then I could go ahead and adjust my beliefs based on those lists as necessary.
Sadly I don’t see any posts like this. Presumably this is because of the social convention where you’re expected to back up any public belief with arguments, so that other people can attempt to poke holes in them. I find this strange because the arguments people present rarely have anything to do with why they believe those things, which makes the whole exercise a giant distraction from the main point that the author is trying to bring across. In order to prevent this kind of derailment, posters tend to cover their arguments with endless qualifications so that their sentences read like this: “I personally believe that, in cases X Y Z and under circumstances B and C, ceteris paribus and barring obvious exceptions, it seems safe to say that murder is wrong, though of course I could be mistaken.” The problems with such excessive argumentation and qualification are threefold:
By contrast, terseness makes posts more readable and makes it less likely that the main point is misunderstood. So if we as a community could just relax the demand for argumentation and qualification somewhat, and we all focussed on debating the main points of posts instead of getting sidetracked, then perhaps experienced rationalists here could write nice and concise posts that give clear and direct answers to complicated questions. Instead, some of the sequences are so long and involve so many arguments, counter-arguments and disclaimers that I feel the point is lost entirely.