People by default will be excited about stuff that's "good news". But the community is pretty big, so there will usually be a set of people that see an issue, and comment on it.
So in the end many things coming through will be negative or cynical. The fact that people tend not to just comment "neat" but will write the paragraphs of research saying how X is wrong doesn't help in the impressions. But for things like research papers, most people tend to just comment on the contents.
I think the reason so few companies on HN have this reputation is because many of these companies are problematic. Google is the symbol of ad networks. Facebook does a bunch of privacy-busting stuff. Amazon exploits many workers on the lower end and it's not certain that their monopoly will be benificial in the future.
If anything I think it's an indictment on the kind of company that succeeds in SV (and, well, capitalism). I think we all remember old "do no evil" Google financing a bunch of awesome stuff. And when they turned out to be like every other company.... well that sucks.
Inversely loads of smaller businesses tend to get a lot of good press on this website, because people can see the passion and the "not about quarterly revenue" attitude.
Everyone, regardless of field, can relate to rockets and cars.
Far fewer people would relate to, say, optimizing insurance benefit payments.
SpaceX is cool because there's a really good chance they'll be more successful at missions to the moon and Mars than NASA has been recently. They're both forward-thinking companies.
Not saying they aren't doing good things. But they get more credit than the good things they're doing would merit. And probably more than even effective PR can explain.
Doing this is way more exhausting than cheering them on (see SpaceX, people have fun in those posts). "Oh this company is doing well and I'm jealous"? Seriously don't know how that's enough.
There's a slightly less nefarious interpretation which is that people have different values, and disagreement happens.
So many people are extremely happy to see other people's success here, for example Lyft here. But what you might think is unimportant virtue signalling is something other people actually care about, sometimes deeply