Initially HTML was less about the presentation layer and more about the "document" concept. Documents should be self-contained, outside of references to other documents.
I still think this is the best web. Either you are a collection of interlinked documents and forms (manual pages, wiki,...), or you are a full application (figma, gmail, google docs). But a lot of sites are trying to be both. And somes are trying to be one while they are the other type.
One document == one HTML page was never the idea. Documents are often way too long to comfortably read and navigate that way. Breaking them into sections and linking between them was part of the core idea of HTML.
Includes are a standard part of many document systems. Headers and footers are a perfect example - if I update a document I certainly don't want to update the document revision number on every single page! It also allows you to add navigation between documents in a way that is easy to maintain.
LaTeX can do it. Microsoft Word can do it (in a typically horrible Microsoftian way). Why not HTML?
