Not all of them do, of course; nevertheless, when it comes to novelizations, suckitude is the way to bet. Most of the time, though, it isn’t really (or at least, not entirely) the writer’s fault.
Reason number one: The writer probably didn’t have that much time to work in. The publisher wants the book to come out at the same time as the movie, and the studio doesn’t make the book deal until fairly late in the game (because in the grand Hollywood scheme of things, the novel tie-in is roughly as important as the Halloween costume and lunchbox rights. Or maybe less.) This leaves the writer facing the directive, “We don’t want it good. We want it Tuesday.”
Reason number two: The writer probably didn’t get a copy of the actual movie to watch before writing the novelization. The studio doesn’t give those out to just anybody, and novelists aren’t even anybody. The writer would have gotten a copy of the screenplay for the movie; if the writer was lucky, it would even have been the version of the screenplay that actually got filmed, and not some earlier — and superseded — version.
Reason number three: The novelizer has to please not only his or her editor at the publishing house, but also the person at the movie studio who’s in charge of maintaining consistency and creative control. This effectively prevents the writer from doing anything innovative or unusual with the material.
Reason number four: A screenplay is a bare bone to make a good soup with. It’ll run, typically, about 120 pages, of which a lot is white space. A knock-down brawl or an epic swashbuckling chandelier-swinging duel may be set down on the page as just “They fight” — presumably on the grounds that the second unit director is going to be handling that sequence and will have his own ideas. (The novelizers will count themselves lucky if the chandelier bit gets a mention, because viewers of the movie are going to remember it, and will complain if it doesn’t turn up in the novel.) In any case, the fictional form that matches most closely a film in length isn’t the novel, it’s the short story or novella; but you don’t see novella-length tie-ins crowding the bookstores. If an author has been charged with making an 80K or even 120K novel out of a standard-weight screenplay, and if he or she isn’t going to be allowed to make up additional story material to fill things out, then the only alternative left is going to be shameless padding.
So the next time you’re reading a novelization that isn’t one of the rare handful of actually pretty good ones, pause a moment and spare a kindly thought for the writer who strove to give the publisher and the movie studio the very best novel that they could get by Tuesday.