I must have missed your point, isn't the answer obviously that HTML is very, very limited and intended as a way to markup text? Semantic data is a way to go fur

I must have missed your point, isn't the answer obviously that HTML is very, very limited and intended as a way to markup text? Semantic data is a way to go further and make machine-readable what actually is inside that text: recipes, places, people, posts, animals, etc, etc and all their various attributes and how they relate to each other. Basically, what you are saying is already rdf/xml, except that devs don't like xml so json-ld came along as a man-machine-friendlier way to do rdf/xml. There are also various microdata formats that allow you to annotate html in a way the machines can parse it as rdf. But that can be limited in some cases if you want to convery more metadata.