That's because you're looking at it from the perspective of what the developers of those news websites want, rather than what the users of those news websites want.
The users want to get a document, not an app. That's what the web was made for, documents.
As a user I want to be able to save the page and read it later.
Admittedly even with .war format and other attempts, this was possible only for a brief time. Now we have a crop of readability converters because even print to pdf is useless most of the time.
The users want to get a document, not an app. That's what the web was made for, documents.