Satisfying endings are subjective to the individual taste of readers. What you or I may find unsatisfactory, may be just fine with other readers.
If we are talking about commercially published books that have gone through editing, then the editor and author have more than likely discussed the ending and if its satisfying. A commercial publishers editor pretty much knows what works and does not work for their readership, based on pass experience.
Now, if we are talking about self published books that have not had the privilege of a commercial editors experience, then I would more than likely agree that many have endings that fall flat for one reason or another.
Of course, the writer has the final say and many times, they just run out of steam and want to end this particular story. Well, if there is no one there to talk them through the ending, then they are at risk of the failed ending. Or not, depending on how good they are...
I remember reading an article many, many years ago in which the writer was interviewing Jerry Pournelle. According to Jerry, while he was writing Janissaries, his editor kept pressuring him to finish the damn book as he kept send it in piecemeal with changes. The editor resolved the issue by making it into two books and Jerry was not happy with the endings of both. So here we have an author who did not know when to stop, because to him, the story was still left unfinished. But the editor had a deadline and resolved the problem to their liking. Me, I had no problem with the endings of both books and never noticed what Jerry had pointed out in the interview.
I sometimes also wonder if as a writer I am too focused on what other authors fail or succeed at and therefore I am too subjective. Or is that objective?...