SRI/hashing works for static content. Though it's worth mentioning it's a SUB-resource feature (images, scripts, etc.). It doesn't work for hyperlinks to other pages. Even if it did, it's a different use case.

Say I link to an article by Author A that has comments in it (or even a footer, relative timestamp, sidebar, etc.). Hashing won't work as the page is always changing. I want the link to always go to Author A but I don't care if the content changes. That's the sort of use case signed webpages and hyperlinks with enforced authorship covers. It's less about what's on the page and more about who created it.