> Not being able to find things later is, well, a problem for later, so it gets deprioritized vs the problems of now.

Who said anything about finding things later? I want a forum because, for niche topics/questions, slow+indexed is the only way to hope anyone will be able to answer my question at all.

I can ask a question on a forum and maybe be answered a week later, by someone who's looking through older open threads under the expectation that they can still be of help; or I can ask a question in a chatroom and the people who know aren't online, chat continues, when they come online they've missed it, and so I get my response never.

It's a key difference in assumptions, mostly due to different etiquette.

When you solve a problem you asked about in a forum thread, you tend to say that. So a forum thread that remains open with no replies is effectively "left hanging", with the author still likely to be interested in an answer. So, for the people who offer help in such forums, there can be value in looking through old open forum threads / solving "cold cases."

Meanwhile, chat messages are sort of "drive-by" things, where you'll write a thing, and then, if nobody responds, leave and never come back. You were hoping for an immediate response; you didn't get it; so you gave up. There's no expectation of someone ever coming back, or seeing a response given much later; so people don't try to offer them.