Because it's HyperText, the main idea is that you link to other content, so this is not a weird feature that is being asked for, it's just a different way of doing the whole raison d'etre of the tech. In fact the tag to link stuff is the <a> tag. It just so happens that it makes you load the other "page", instead of transcluding content, the idea is that you load it.

It wouldn't make sense to transclude the article about the United States in the article about Wyoming (and in fact modern wikipedia shows a pop up bubble doing a partial transclusion, but would benefit in no way from basic html transclusion.)

It's a simple idea. But of course modern HTML is not at all what HTML was designed to be, but that's the canonical answer.

The elders of HTML would just tell you to make an <a> link to whatever you wanted to transclude instead. Be it a "footer/header/table of contents" or another encylcopdic article, or whatever. Because that's how HTML works, and not the way you suggest.

Think of what would happen if it were the case, you would transclude page A, which transcludes page B, and so with page C, possibly recursively transcluding page B and so. You would transform the User Agent (browser) into a whole WWW crawler!

It's because HTML is pass by reference, not pass by copy.