Seems to be an incredibly common problem. TV writers don't seem able to make good stories, perhaps the skills needed are quite different. It's notable that nearly all high budget TV serials in recent years are adaptations of books. Knowing the books were successful surely reduces the risk, but then having found a winning formula the TV firm wants the series to last forever. They run out of book material and then the story goes off the rails as the TV writers can't match it. GoT being a classic example.

I suspect the issue is timelines. TV projects are managed with tight deadlines because they expect a new series within a year. A book can be released more or less whenever the writer wants, as they answer to nobody unless they signed some sort of multi-book deal which is rare. Writers take more of the financial risk on themselves, whereas in TV it's the TV firm taking all the financial risk. So you get whatever plot they could come up with in 6 weeks or however long they have, and it can't match the quality of a story where perhaps the writer spent a year on it.