It's either waiting endlessly for the next book, or they just keep pumping out new ones with no end in sight. There are a few series out there that I would love to get into, but they have way to many novels for me to finish or I just can't chance the writer getting writer's block.

 

I think it's because the author either has too much money or not enough.

 

It's because they either have too many plans or not enough.

 

Endings are hard.

 

I mean...does it happen that often? I can certainly think of some examples, but it's not like there's a shortage of complete series either.

 

I think a lot of the time authors just get burned out on whatever fantasy world/story they're writing in. After a while, the passion sort of runs out and what you're left with is just work. Then, if you're in a position like George R.R. Martin, I'm sure there's a lot of temptation to just say "Fuck it, I'm already rich and famous, so why bother slogging through the rest of this?"

 

Fantasy authors do finish their series. All the time. More often than not, by a sizeable margin. There have been a few big names recently who've failed to deliver, but these - Rothfuss, Lynch, GRRM - are the exception and not the rule.

They very often will go on to write sequel series in the same world, but that's not the same as leaving the third book in a trilogy missing.

 

I dunno, compared to almost all fanfic authors, published authors mostly get stuff done.

 

Without wishing to be overly pedantic (on DLP? Perish the thought!) there's a difference between a series not sticking the landing at the end, and a series not finishing at all, for whatever reason.

Narnia, Harry Potter, The Dark is Rising, two Malazan series, Codex Alera, the Belgariad, Earthsea, Book of the New Sun...all complete series, four books minimum, all gathered from a quick look at my shelves and a couple of minutes thought. I'm sure there are others - and personally, I'd also include trilogies, but I'll stick with your definition for now.

 

That's a pretty low bar you're setting. 90% of published authors could fail to finish their series and they would still be doing better than fanfic authors on average.

 

It seems very unfair and not really logical to include series with "bad" endings in a discussion about a supposed problem of fantasy series not being completed in a reasonable timeframe. As others said writing a satisfactory ending is hard and I would argue that with a long series a lot of readers develop their own ideas and interpretations of the story and characters which make accomplishing that even harder.

And I really don't see much evidence of there being a huge problem with unfinished or horrendously slow updating series in the fantasy/sci-fi genre. Sure there are some big names that have had significant delays if not cancellation but the vast majority of series get finished/continued within a reasonable time-frame. And from my experience a lot of the cancellation/delays are either caused by lack of readers making it uneconomic to continue writing a series or at least very hard to get published or because the author suffers from mental health issues which is just bad luck (and maybe a sign of our times and the profession).

And while you appear to view long series as inherently negative I personally am of the opposite opinion and prefer long books and series as that means I have more content to consume/read and thus a chance to spend more than a single evening or two enmeshed in the world and its characters.