“…the question here is, seriously, what is the point in making subpar, incomplete, movies…”
I get this isn’t your literal question, but nobody (or almost nobody) sets out to make a bad movie, whether original or an adaptation. To answer your more specific question, it comes down to essentiality or practicality: Tom Bombadil—as many Lord of the Rings readers will tell you—is hardly germane to the plot of the book (I find he kills the story just as it—finally—gets going), and easily removable from a film adaptation, where length is key; as to Jurassic Parks’s raptor pen, that will have been as a result of special effects technology coming up against the budget, where the nascent CGI technology and practical effects of the time would have been far too expensive to justify a scene which is, as we see, largely inessential to the plot.
Agreed—I’ve read the synopses of those three books and they’re all folk horror, maybe supernatural. ‘Cottagecore’ may be a lifestyle movement, but applying it as a genre of literature sounds like a bit of useless preciousness to me.