You've asked an interesting psychological question.
I think first off, people approach difficult situations differently. Some people seek the optimistic view, some feel better prepared if they look the worst case scenario in the eye with frankness.
That's perfectly reasonable, as human beings we're a sum of our experiences so of course everyone's approach is going to be different.
However, it would be naive to deny that there are some people who actively seek to spread anxiety and despair. They may not be the majority in any group setting, but they exist. We see them on covid threads, Brexit and Trump threads too. They're all over the internet, in fact. They quite often post evidentially suspect information that presents things in an unrealistically poor light, and then fail to produce any data to back up their assertion.
Why do they do this? There's probably a range of reasons. I think for some people, it's a tactic to (badly) manage their own anxiety. If they can spread some of it, they're not suffering alone.
Some people just want to stir the pot. WUMs who get a kick out of frightening others. Some people are angry and hurt and want to wound, and they're usually cowards so the safest way for them to do that is on an anonymous forum.
Then there's the misogyny angle. MN is a forum populated largely by women; don't underestimate the amount of weird, angry men who come on here in disguise to post stuff to 'wind the wimmin up' in order to screenshot it for posting later on the murkier and more red-pilly parts of Reddit or Twitter.
It is 'a thing' in real life too. My grandmother was like this: you'd barely get in the door and she'd be regaling you with dark mutterings about the perils of bacon. She was a ghoul about other people's sufferings too; always on about the neighbour whose son died, or the woman in a car accident, or what went wrong with Connie's 'downstairs operation'. With her, it was a sense of fear that had become twisted; 'if it's happening to them, it's not happening to me.'