Either the author knew of prior work that argues the same thing and they ignored it, or they didn't know. And if one writes a 1000+ word article premised on this idea, wouldn't one be presumed to know at least in which century the idea originated from?
Arguably these kind of blog posts should be more subject to such criticisms, because the blog posts purport to "teach" the general public about a concept in an authoritative tone (or at least the author seems to pose as knowledgeable in the subject), while for academic papers, everyone who actually reads the paper knows where the ideas came from anyway and it's mainly an issue of attribution (and maybe about fairly distributing the citation count...)
You're asking why casual comments from amateurs made for fun on the internet shouldn't be held to the same standard as those made by funded career academic experts writing for other experts over months and meant as part of the permanent record of the field?
Personally, I think that's a bit like asking why a friend having you over for dinner isn't always an elegant 7-course meal with wine pairings. I guess you can expect that if you want, but to me it sounds like a child expecting to go to Disney every day: ignoring the economic realities of the situation is a recipe for eternal disappointment.
If the blog is a personal collection of "today I've learned" then different standards apply then "my blog is my research publishing platform".
Both are valid blogs. The fact that it ended on HN front page didn't mean it was targeted towards this audience
This should be expected out of everyone. If you don't respect the reader enough to do this, why should we read your posts?
I think papers should be retracted for not citing prior art, even if you weren't aware of it.