You could easily make an HTML viewer that pages content, just as some ePub viewers allow you to scroll through the content.

Yet something like ePub/MOBI is definitely better for books. As others have mentioned, it bundles the content together. It goes beyond that by restricting the markup to a subset that ereaders can reasonably handle, and adding features like a table of contents.

PDF seems to do everything that ePub/MOBI does and adds consistent formatting. The problem is that it's a page description language. Describing a page doesn't work that well when the page can range from less than 4" to over 10".

It is possible to blend the two. That's sort-of what TeX does. Yet anyone who has used TeX can relay the horrors of describing a document that renders properly at one page size, never mind multiple page sizes.