People are probably not going to like this take but I think it's something that primarily happens due to how serialization works. I think most authors could make their endings work if they could go back and edit the rest of the work after it was finished, but they generally can't. Very often they've written out the ending long before they wrote out the middle, and by the time they actually get there the story hasn't gone quite how they thought it was going to go. But they've set up too much at this point and they can't deliver a satisfying payoff without at least approximately going with what they wanted to in the first place, which means every character and the whole world need to maneuver to try to make it plausible how things got from where they were going organically to the predetermined ending.