A few years back I joined HN and was so intimidated by all the smart people here that it took a long time before I started commenting. Now, if you're not paying attention it's easy to mistakenly assume that the smart people "know better" and will comment with their opinions on how they'd do XYZ better. A lot of the threads I saw back then had people giving contrarian opinions and getting lots of attention for it. So for the newbie it's most likely easy to assume that's how things are done around here and that's how you prove your intelligence/expertise. But the thing is, what often looks like a contrarian view is often a question or an alternative way to do XYZ that isn't really saying XYZ is bad or wrong at all.
So I think it's a case of monkey see monkey do mixed with a misunderstanding of popular comments, who their authors are, and missing context. Everyone wants to be the smartest person in the room. HN can be an intimidating place because of how seriously exceptional a lot of people around here are. Hell, I still feel like a moron half the time when I comment. Egos get in the way of accepting that instead of being the smartest you should learn from the smartest and seeing that you don't have to say anything at all. In fact, saying nothing is often the smartest thing to do. How does that saying go? Something like "Shut up and let people assume you're stupid instead of opening your mouth and confirming it"?