Phillip Pullman published a well-known trilogy from 1995-2000 called 'His Dark Materials', comprising Northern Lights (aka The Golden Compass), The Subtle Knife and The Amber Spyglass.
The point I wish to make about this series is that it starts very well, and ends very badly. The contrast is extreme - the first part of the first novel is very good, and by the end of the third volume it has become very bad.
This interpretation of His Dark Materials is not really a matter of opinion, but as close to objective fact as you can get in literary criticism - anyone who doubts should read John C Wright on the technical aspects; and the ideological reason for these gross errors and incompetencies.
But bad endings are not unusual - indeed, it is quite normal, statistically common, even fashionable and mainstream - for books, plays, movies, and TV plays and series to start well and end badly.
But why should this be? Why do they, on the one hand, start well; and on the other hand, finish badly?
I think the answer is quite simple. Great works are written with the assistance of genius, which can be understood as a visitor who will only stay if treated well.
Many more talented writers are visited by genius than succeed in completing works of genius, because most writers betray their visiting genius before the work is complete - the genius flies away, and the work must be finished without it; and talent without genius is comparatively a very poor thing.
So, Phillip Pullman was a talented writer who was visited by genius and began Northern Lights; but he was dishonest in the way he used this gift - so away it flew and he cobbled together the rest of the trilogy on his own - getting across the 'message' which meant so much to him, but making a an artistic pig's ear of the writing.
JK Rowling is different. She was visited by genius when writing Harry Potter, and it sustained her through seven volumes - and the last volume, including its ending, are wonderful! The best thing in it (probably).
There are signs that, part way through, Rowling became personally dishonest and corrupt - probably due to the temptations of fame - but she kept this out of the books. However, when she finished Harry Potter, Rowling embraced the dark side, and away genius flew - her work has waned even as her commitment to political correctness has waxed - and she has been caught in several blatant lies about her life and work. I would not expect her ever to write anything really good ever again.
However, throughout HP, Rowling was true to her visiting genius - it stayed with her for the duration, and she was therefore able to complete a great work.
And of course the benchmark classics all end well, else they would not be real classics. The Lord of the Rings, the Narnia Chronicles (despite extreme digressiveness en route), The Wind in the Willows (despite major incoherence and a detachably episodic structure) - all of these have deeply satisfying endings.
No author is so great as to overcome the need for a good ending. When George Bernard Shaw tried to end Pygmalion with an anticlimax, the actors when on tour secretly substituted the dramatically-implied-and-required 'comedic' ending (ie implying marriage between Higgins and Eliza). This, to Shaw's extreme annoyance! - but they would not stop doing it, because the audience response told them what was right. And the highly successful musical adaptation (My Fair Lady) made exactly the same change.
So, even so great a playwright as Shaw was not immune to the absolute requirement for a good ending. Even the greatest of all - Shakespeare - has to bow to this imperative...
All Shakespeare's best plays (best, that is, according to the consensus of playgoers through the ages) end well; and the ones that don't end well are not regarded as great.For instance, Measure for Measure is shaping-up superbly for most of its length; but the play has a truly terrible ending - so it never has become a part of the standard dramatic repertoire (despite having the best 'strong' female role Shakespeare ever wrote).
This, then, is a possible reason why too many books end badly; and why so many other narrative forms end badly too. It comes down to the abuse of visiting genius.
Bad endings have indeed, become a feature of modernist writing over the past century. The inability to finish a book, play, movie has been covered-up with nonsense about the sophistication of an 'ambiguous' ending, or 'deliberate' anticlimax, or the desirability of 'dark', 'subversive' conclusions; or the need for some kind of radically transgressive and expectation-thwarting finish (to educate the audience).
But the fact is that it is an extremely difficult thing to end a narrative well - and I would regard supposedly deliberately ambiguous (etc.) endings as a fake; an excuse to cover-up incompetence and failure.
Bad endings are so common nowadays because corruption of writers has become so common as to be nearly universal. When visited by genius, the writer is not grateful, does not perceive that this entails a duty to be truthful to his inspiration - but instead he tries to use the gifts of genius to pursue to fashionable ideology, or to ride some personal hobby horse.
Corruption leads to betrayal of genius - usually by dishonesty; and the cause of dishonesty is usually some brand of Left Wing/ radical politics - which the writer or artist places above the truth of art.
Dishonest art cannot be great; and an habitual liar can only be a great artist when he is (nonetheless) utterly truthful in his art - however badly he behaves the rest of the time.