I find it hard to get the point of novelising a movie that's based on a novel...
I generally resist that as well, although, yeah, there are some cases where the film/TV show is so
different from the original than it almost makes sense. There's a new edition of Dracula
(the original novel by Bram Stoker) that has the guy from the new NBC TV series on the cover, but anybody who picks up that book, expecting anything like the TV show is in for a big shock. That's a case where it might have made sense to commission a novelization of the TV show instead. (Too bad Fred Saberhagen is no longer with us.)
But, yeah, I've never really seen the point of hiring, say, David McIntee to rewrite The Island of Doctor Moreau
where there's already a perfectly good novel by H.G. Wells.
And from an editorial standpoint, it's frankly easier to slap a new cover on an old book than produce a new book on a crash schedule . . . assuming you can get permission from the studio.
There's another consideration, of course. If you're dealing with a public-domain classic, the studio or publisher may want something they can sell exclusively
. The problem with a doing a movie tie-in edition of, say, Dracula
or Pride and Prejudice
or whatever is that everyone in town can put out their own edition to cash in on a new movie--and there are probably already multiple editions in print. Having the movie cover is a plus, but there's still going to be plenty of competition in the marketplace . . . .