HTML and CSS are built for web pages, which means they don't really have a strong concept of page size. If I want to build a table with headers that repeat on every page of a printed document, it's an absolute pain to do it with HTML and CSS.
You need something with constructs that can express pages, repeated tables, headers and footers, stuff like that in a much more native format.
Sorry if I wasn’t clear. I didn’t mean “Make it work with HTML and CSS today”. I meant, Why couldn’t HTML and CSS be used as a basis for a PDF-like format? New features are constantly being added to HTML and CSS. In 5 to 10 years, maybe we’ll have a revamped powerful paged layout that will have all the features that you mentioned.
So my point is, why create a new format when you can enhance HTML and CSS?
You need something with constructs that can express pages, repeated tables, headers and footers, stuff like that in a much more native format.