This is one I've wrestled with, too. Very occasionally, I have seen exceptions to the politeness that is the norm in this space, such as some of the exchanges on this post from Jay, for example. It's rare enough that I was quite taken aback by it.

By contrast, the comments on this YouTube video degenerated into personal attacks between commenters.

There is of course the issue of anonymity - the main instigator of vitriole was one who used a nom de plume and took the stance of cynical critic from the off. Like you, I have given up reading the comments on YouTube videos, now.

But I wonder if another factor might be the lack of buy-in to a common purpose. For example. I used to belong to a weight loss website, which had discussion forums. There were occasional spats there, but they were mostly due to misunderstandings resulting from the limitations of textual communication. For the most part, the conversations were mutually supportive, positive, encouraging and very we're-all-in-this-together.

I think there is probably still a lot of cynicism out there towards learning in the workplace. Not everyone is sold on the idea of "let's do this as well as we possibly can, for the good of us all". Many people are still standing back with "Oh yeah?" oozing out of every fibre of their beings. I suspect many of the more enthusiastic types are a little intimidated by the eye-rollers, so they hold back.

Just thinking out loud...