It's bad when a writer is tired of a series and has to keep writing it, but there are also the authors who think they're still writing good books, and honestly, they're not. I like mysteries, so my big example of this is Joanne Fluke's Hannah Swenson series. I haven't been able to bring myself to read the last couple of books, because the downhill slide had gotten so bad.
The problem is that they don't have to keep writing it. They can stop. Yes, they'll have to face the disappointment of their fans, but that's what comes with fame. Sadly, it sounds like Harris, Patterson, Fluke, Lackey and the rest are lacking in spines -- they're afraid of disappointing their fans and won't stop.
Author contracts are tricky things, and there are some pretty unethical, mind-bendingly evil things out there that publishers can do. In particular, some contracts say any combination of the following:
1) the publisher gets the "right of first refusal" to the author's next book(s) (the author is not allowed to sell a book to anyone else until their current publisher says "yes" or "no" to it)
2) the publisher owns the copyrights on the character, setting, etc.
3) the publisher can cancel a series at any time, for any reason
4) the publisher owns the author's pseudonym
As you can imagine, the combinations of these can be really messy! If an author has both #2 and #3 in their contract, the publisher can cancel their series halfway through and the author legally can't write any more books in that setting. If you have #1 without sufficient limitations, the author is locked in - if they want to keep a career as an author, they have to keep writing whatever the publisher wants. If they write something else, the publisher can just indefinitely refuse to say "yes" or "no" to it, and the author can't legally try to sell their books elsewhere. If you combine that with #4, the author can't even leave for another publisher without having to start all over building a reader base. (Yes, some of your more rabid readers will follow you to your new publisher with your new pseudonym, but most won't pay close enough attention to figure out what happened.)
Luckily, most publishers aren't too bad about this, and literary agents are there to ensure authors don't sign away their firstborn by mistake. I can't be too hard on experienced authors who stick with a series long after the series has stopped being creative and fresh, though - sometimes they don't have a lot of options (other than a complete career change).