I've actually wondered why a few of them haven't done this already.
I'm guessing it's because they don't see the need. Thinking seriously about it, why would Stephen King ever go indie? He makes millions already, someone else is always taking care of things for him, such as editing, covers, marketing and so on. Why would he leave the world of tradpub behind and try to learn how to indie publish? I just don't see it for any of the top shelf authors. King, Lee Child, Dan Brown, JK Rowling and boatloads of others. They make comfortable livings without doing anything, but writing their books.
Interestingly, Stephen King tried to go indie. Remember
The Plant? Not very many people do, but it was King's attempt to sell a novel online in installments. The explicit purpose was to cut out the middlemen (publishers and distributors). King was ahead of his time in that respect. People weren't used to buying books that way, and even with his popularity, he couldn't sell the concept on his own. Had he waited until KDP took off, it might have been a different story.
As for JK Rowling, when she got big enough, she took as many of her rights as weren't already nailed down and created Pottermore. (I suspect one of the reasons publishers became more rights-grabby is in an attempt to avoid future Pottermores.)
That said, most authors who are perennial bestsellers are happy in their current situation. They are the few people for whom traditional publishing undeniably works. Could they make more as indies? With their enormous fan bases, probably, but there is risk in jumping out of a model that's worked well for them. I also suspect that they can turn down attempts to lock them into more onerous contract terms. It's the newer authors who don't have huge fan bases they might take elsewhere who are more likely to get stuck. They just don't have the bargaining leverage to easily say no to a major publisher.
Such writers would be ripe for a model with some of the benefits of trad and some of the benefits of indie if anyone could design one. I thought Amazon had the right idea with Kindle Scout: trad publishing with more author creative control and reasonable terms, including easy rights reversion. At the time it started, I thought Amazon was trying to snag the future Stephen Kings and JK Rowlings by giving them an alternative to the Big Five. I was wrong about that. Kindle Press, the imprint into which Kindle Scout books fed, ended up as the poor stepchild among the imprints. It gradually did less and less to promote books. Even as Amazon imprints in general were taking a bigger piece of the sales pie, a lot of KP books just sank. They weren't even included in programs like Kindle First Reads and other opportunities that Amazon could have given them easily. Kindle Press also lacked the vision to develop relationships with authors. Even authors whose books did well didn't always get picked up again. Editors had a very narrow, book by book vision, and often picked up only one book in a series, even if the first book did well. We learned during the phase when finalists were getting editorial comments that books could be rejected over relatively nitpicky things that could easily have been fixed in editing (a side-effect of the only-on-round-of-editing process). The biggest heartbreaker for me was the rejection of a book (which wasn't mine) that the editor described as "brilliant." It was rejected because the author didn't seem to know anything about promotion. Isn't that the publisher's job? No wonder that process didn't work.
OK, enough Kindle Scout bitterness. In the future, if someone with vision and patience tried a model like that, it could probably be made to work. Even KS had a few success stories to its credit. Someone looking at the future instead of just the book right in front of them, someone understanding that partnership with authors is better than the servitude a lot of trads are imposing, might really create an enterprise that would take off.
