This has no benefit for the person who has to pay to do the work. Why would I pay someone to mark up all my data, just for the greater good? When humans are looking/using my products, none of this is visible. It's not built into any tools, it doesn't get me more SEO, and it doesn't get me any more sales.
Meanwhile, why would say Mouser or Airbnb pay someone to markup their docs? WebMD? Clearly nothing has been compelling them to do so thus far, and when you’re talking about harvesting data and using it elsewhere, it’s a difficult argument to make. Google already gets them plenty of traffic without these efforts.
Also, I just checked: WebMD is using a ton of embedded RDF on each page. They understand SEO well as you said :)