They can't do that because publishing these days is, when you get down to it, all about supply chain contract management. And in particular, my book contracts stipulate in mind-numbing detail precisely how sales will be accounted for (what percentage of net receipts go to me, or to the publisher, depending on volume and/or discount rates). The contracts make hard-coded assumptions about
how books are sold that can't readily be rewritten. Also, contracts for different books by the same author may have different T&Cs, never mind contracts issued in different years (occasionally publishers try to "fix" things by turning Legal loose on their inherited contract boilerplate, usually with questionable results because they invariably make a massive land-grab and then the bigger literary agents all lawyer up and arm-wrestle for a better deal for their clients).
It's bad enough that at least one of the Big Five (to my definite knowledge, as I work with them) has fired up an internal start-up publisher, mostly using the same editorial team and production facilities, but where the distinguishing characteristic is that (a) they totally rewrote their book contracts to make it possible to do new and exciting stuff, and (b) target ebook sales as a primary channel. (Disclaimer: this is my primary publisher in the US market right now and I'm very happy with the way things are going.)
But when you're dealing with multinational publishing corporations you've got to bear in mind they have tens of thousands of book contracts going back decades which can't easily be dragged around onto a new business model, and they suffer from activist shareholders and an ever-constant threat of hostile takeover (see the ongoing Simon and Schuster takeover by Penguin Random House).