It stands for "markup language", and was inherited from SGML, which had includes. Strictly speaking, so did early HTML (since it was just an SGML subset), it's just that browsers didn't bother implementing it, for the most part. So it's not that it didn't evolve, but rather it devolved.
Nor is this something unique to SGML. XML is also a "markup language", yet XInclude is a thing.
That's why I joked about flamebait, it's hypertext though, aren't anchors essentially a goToURL() click handler in some ways? Template partials seem like a basic part of this system.
> considered to be server-side
Good point! Wouldn't fetching a template partial happen the same way (like fetching an image?)
Some content is already loaded asynchronously such as images, content below the fold etc.
> HTML is really just a markup syntax, not a programming language
flamebait detected :) It's a declarative language, interpreted by each browser engine separately.