I think people want someone to blame because it then makes it preventable. If it’s preventable then they can do something to ensure it doesn’t happen to them or someone they love. They don’t want to accept the reality that sometimes shit things happen and it wasn’t foreseeable and nobody is to blame. If they accept that reality they then have to confront the possibility that it could become their reality.
This is the main reason people do it.
It's also why people 'victim blame' generally. Well if she hadn't been walking on her own in the dark... well if she had left him the first time he hit her...; well if... it's because people think if they follow an arbitrary set of rules they won't be a victim.
But there's another aspect. A lot of people watch soaps and trashy reality tv etc. These shape social narratives and the language people use. Some people speak about real life events using the same dramatic language as you see in soaps or reality TV. Always looking for the 'drama', the outrage, the plot twist or the cliffhanger.
And yes, peope who comment on public online forums aren't generally the brightest.
Many accidents are technically preventable. But humans are flawed and make mistakes, take risks, don't predict possible outcomes, don't recognise potential unintended consequences etc.
Eg someone puts something on the stairs intending to take it up next time they go; they've done it for years and no one has ever tripped over it and fallen down the stairs; they grow confident in doing it over time because nothing has ever happened; one day, someone trips over it and falls down the stairs. Technically, it was preventable because no one can trip over something if it isn't there but their ongoing internal risk assessment had put it at a very low risk because no one has ever tripped before. They hadn't put it there hoping someone would trip over it and the person who fell didnt see it and think, I'm going to trip over that. So it was an accident but it was also preventable.
And in that case, people would he quick to point the finger of blame - well, it's obvious that if you put something on the stairs, someone will trip over it! But if that hasn't happened in, say, 20 years of doing it, the risk feels very low and people will take the chance.