Because in the context of other horror movies, it will be a lot more entertaining than most of the crap out there. At least with Stephen King it will be as original as you can get these days, and if actually believe the writer might be able to throw something new at you, you actually think there might be something unknown in this movie to fear, unlike movies pasted from every horror cliche out there. Moreover, the characters will be real people and not people who act just as fantastically as the events that are happening to them, allowing you to really connect their fear with yours.

Sure, it will still be anti-climactic, because any time the metaphysical, malevolent force comes to a show-down with the physical protaganist, they must interact, putting them on the same plane of existance and negating the metaphysical nature and properties of the evil, which destroys the fear of the unknown the movie was built on. (Incidentally, that also works as a little proof against the concept of metaphysics in general, not to ruffle any feathers.)

But then, if we temper our expectations by understanding that we're really there to enjoy the build up, we can enjoy it for what it is, and my guess is that Frank Darabont has nailed this build up. In fact, sight unseen, I would submit that Frank should be given more money and be solely in charge of adapting King's work for the rest of his career, although I am damn curious to know what such a talent could do to an original piece of work, considering he hasn't done anything that wasn't scripted by Stephen King yet.