When was the last time you saw a site with a JSON API providing metadata, like the json-ld for a product on an e-commerce site? Or an API just for the open graph data? How would you even discover these APIs for sites that you don't own?
It's also worth noting that very, very few JSON APIs today are actually REST. They rarely include all the context needed, and in general JSON is much less useful than XML when you're talking to other APIs that you don't own since JSON can't easily describe the shape and datatypes of the content.