Your math is a bit wonky and you are missing some aspects of the traditional publishing contract.
The traditional publishing deal involves the author getting an advance (cash up front) which they do not have to pay back unless they fail to deliver the manuscript. So the publisher takes on the risk that the book won't cover it's advance. The publisher also manages book production and distribution and publicity, to do those things the publisher keeps staff on hand in some fairly expensive office space ( publishing happens in Manhattan, or used to ). All of which add up to a pretty good deal for authors whose work is salable.
With Amazon or Apple, the author takes on the risk, and all the costs of production ( poorly edited ebook only editions aren't going to sell as well as actual books ) and all the costs of marketing and promoting their work. The only thing that's taken care of is distribution; for which they give up 30%, not the actual cost of distribution.
So the new deal is riskier and less certain ( if you pay an editor, a book designer, a publicist etc. ) you might wind up losing money.