My friends and I have noticed (and frequently complained about) the way that some authors' books seem to degenerate over their career-Rowling and Pullman leap to mind... Anne McCaffrey seemed to have a bit of this problem too, back in the day. Do editors just stop editing them because they're guaranteed cash cows? Or is it a problem with authorial hubris? On the other hand, Rick Riordan's latest book still seemed pretty tight. And Terry Pratchett has improved over time.... So... is a lack of editing for popular authors an editorial or authorial decision? Or is it the whole "secret ending can't have too many eyes" problem?In my experience, it's mostly commonly a combination of authorial hubris and publisher gutlessness.
It is a fact of life that authors, who may spend a great deal of time lovingly reading fanmail and googling themselves, end up with a warped opinion of their own talent. This is how a charming author who once knew her own mind but valued others' informed opinions becomes someone who doesn't just think her editor doesn't understand her, but doesn't see why she should bother trying to explain herself. She is brilliant! She is an artist! No wonder we don't get what she's trying to do--she is operating at a whole different level! (...of egomania.)
Now combine that with a medium-high level of career success. The editor who looks at the newest manuscript from this person wants to send it back and say, "I love your writing, but there's no character development or plot arc in this. You can do better." But the editor knows that (a) if the manuscript was published as-is, people would still buy it. A lot of people. And (b) if the editor says no to this, there's a fair chance the author would just turn around and sell it to some other publisher. Then that other publisher would have the mediocre book and the pile of money, and the original editor's Sales and Marketing people would be Highly Displeased that those sales were not theirs. Highly. Displeased.
Who has the guts to say no to money? The truth is, not a lot of people, especially in a business that's low-profit for pretty much everybody involved. And, more's the pity, those of us who want to Just Say No are under some serious pressure to say yes. You can get kicked out of school for not taking these drugs.