In the specific case of html and css I agree that it rarely works. There are a few examples like subreddit css, but in general html does to much layout to meaningfully separate content from layout. Xml and XSLT is a much better separation, with Xml containing pure content and Xslt transforming that into XHTML with CSS for presentation. But that never got much adoption.
In other domains separating content from presentation works well. But in web development the incentives don't align with that idea.
Agreed. The real answer, as you noted, is that presentation really is HTML+CSS. You can join it with content in JSON (or XML if you want) via client or server side templating.
It actually gets you pretty close to MVC or whatever MVVC is maybe? Is that still a thing?
In other domains separating content from presentation works well. But in web development the incentives don't align with that idea.