The Big Five are all about supply chain management contracts. That's all. Everything that can be outsourced is outsourced.
But this leaves them vulnerable to obsolescent boilerplate contracts horked up by an ossified legal department, which get in the way of business innovation.
Some of them are working around this, either by firing off contract updates for backlist titles to be signed by their authors or assignees: others by incubating internal startup ventures with from-scratch redesign of their workflow.
Example: Tor.com is an imprint of Macmillan, and its chief executive is Fritz Foy. It goes back about a decade and its front end is a very successful SF/F media website. Tor is also an imprint of Macmillan, and its chief executive is Fritz Foy. It goes back about 40 years and its website is ... not its best aspect. In fact, Tor.com's entire editorial staff also wear Tor hats from time to time, although their marketing team are different. But the contracts might as well come from another star system. Tor's legalese focusses on the traditional trade fiction distribution channels and would you please sign this paragraph about microfiche and gramophone rights just in case we still use those formats, while Tor.com assumes ebooks have priority and by the way there'll be a hardcover issued via Tor as a distributor, and maybe a trade paperback.)