I know what you mean, I really do. But think of an interactive document that keeps updating itself with remote data. That sounds really cool to me.
Isn't epub a zip file of a bunch of html docs, metadata and images?
If that's the case, you didn't have a genuine EPUB to start with. To meet the spec it needs to be in a container (a renamed ZIP file) and have a handful of related metadata and navigation files alongside it.
That said, the actual text of the book is done by HTML/CSS, but within the EPUB container file.