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Because it is designed as a standard. There is a specification of what must be supported by ePUB.
You could probably get the same (or more) features if you just create an HTML file with CSS, and a text file with metadata, and package everything in a RAR file, but then you'll have to find a reader that can deal with that, and other people might use a different format (different metadata, TeX instead of HTML, tar.bz2 instead of RAR...). Having something standardized is a good thing.
It is better than the sum of its part, because it is not only the sum of its parts, but the interaction between them and the security of the main format being fixed.
An important reason (in my view) to prefer ePUB over other formats is that ePUB books can very easily be edited and modifed without altering the rest of the book or degrading it in any way. At least DRM-free ePUBs.
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