Why you just did?

Precisely. I'm contributing anyway (by commenting, breathing, farting), so why should I do it even more?

Blogging is far preferable to commenting in a public forum because I don't have to see it unless I visit.

To use your own analogy: commenting on Reddit/Youtube/HN is like farting in public. Blogging is like farting in your own home.

There is a significant nuance: blogs are most likely indexed by google. Comments are invisible. So blogs are generating much more noise, obstructing an access to information much more dramatically than even a 5000-strong comments thread could ever do.

Comments are most certainly indexed by Google. They are not usually ranked highly. And crappy blogs are also indexed and not ranked very highly.

Both generate comparable "noise", except that there are a lot more comments.

Then why, when I search for a solution to a specific problem (e.g., googling for an error message, etc.), I most likely land in some blog full of ramblings with no solution whatsoever? And often the real solution is hidden somewhere in comments on SE, never listed in any sane google query.