Well, if they mostly sell on line, and via Lulu, there's no real need for them to own their own presses, not that I said that they do own any, I said it's one way of not paying a middleman. Murdoch's newspaper empire do also publish books, though. HarperCollins.
I don't know about that ten you mention, there seems to be a Big 5 (the above being one of them) and as I said, most of the big parent companies own many many Imprints, many of that 6,000 which could be under the wing of the big guys. https://en.wikipedia.org/wiki/HarperCollins#Imprints here's another with many Imprints https://en.wikipedia.org/wiki/Hachette_Book_Group#Current_publishing_groups_and_imprints
Anyhow, even some massive newspaper printing machines are now POD. Now there's little need for printing plates, they can change what is being printed as the machines continue to print. The outlay for such machines is massive, though.
POD only really means a printing machine that can have files fed to it, rather than have to stop for plates to be changed. (BOD is a bit different. They have to include binding machines.) And most modern printing machines are of that kind now. What is being printed is irrelevant. Not far from me is an art gallery that sells prints. They replaced their frame-printing section with a wide format deskjet printing machine.
Traditional printing, especially offset printing, which is largely the standard, is infinitely cheaper per unit when printing more than a hundred or so copies of a book. This is even more dramatically true if any color whatsoever is involved. There is a very long list of other advantages over digital printing.
Advantages overcome by the rapid onset of technology. Which new tech is always initially expensive to buy. I recall the head of Personnel at British Leyland (who were always on strike) telling me how he showed the union Shop Stewards a film of a Japanese car factory (no humans in sight!) and told them that film is ten years old. "Imagine what we are competing with now." Within twenty years the UK car industry was almost deceased. The same happened in the USA did it not? The advance of technology is unbelievable, and often takes companies by surprise. Currently just the speed of POD machines is the problem. Regarding newspapers, some of the very largest papers are using webfed inkjet presses,
Is that not what I said? But at one time they all did not use any digital method at all, until a local newspaper publisher built a new fully digital newspaper print factory (not far from my house, in fact) causing most of Fleetstreet's workers to come out on strike, when the publishers there said they would eventually introduce the same machines. As I said, Rupert Murdoch has invested (very very heavily!) in a fully POD newspaper printing press in London, it still prints so fast the paper is just a blur, and does not have to stop to change what it is printing. How soon will the rest have to follow?
but newspaper and book publishing are two very, very different things.
The all still have to use printing machines, if they own them or not.
I have many examples of recently published books by mainstream publishing houses that are obviously laser of bubblejet printed.
The future comes faster than you think, especially nowadays. As does what is in a lot of SF