Earlier quoted context omitted.
That is because people who do HTML markup are usually not developers. They are either wannabe developers or just incompetent people hired to reduce costs. They would never waste their time on reading something complicated as W3C standards and understanding concepts like semantic markup.
I'm curious about who read w3c standards, even on HN. I don't. Even MDN (which I consider as fork of w3c for humans) can be quite confusing so let's not judge people on this.
imo, there are a lot of different paths to being a good developer. Personally I find scouring over the standards or some dry reference site to be about the most painful way.