Great books make "epic" films?

Coming on the heels of Harry Potter(Personally, I didn't fall under it's spell), the Lord of the Ring and War of the Worlds and the upcoming Narnia, Memoirs of a Geisha and Tristan and Isolde*, I asked myself why is Hollywood so immersed in the "epic" style film based on popular books. Some of the books I have mentionned should be "epics", the others should not. Not only is it horrendously tedious to sit in a theatre for 2 and a half hours to watch 45 minutes of a movie, it's unsettling because some of the above books don't have the story to support the style.

No doubt there are problems doing direct translations from book form to film. But as the Lord of the Rings has shown, you can follow the book almost dead on and still have a great movie. The devil is in the details, you know.

Memoirs of a Geisha and Tristan and Isolde don't seem like the typical epic films but they both have the "epic" visual style. Narnia has buzz but not as much as I would have liked to have seen because the film looks great. And yes, there are Christian overtones, yes the cast is mostly white and the issues discussed in the story are binary, good or evil, get over it people its a story written in 1950. These facts always start some witch hunt or protest or greivance to the MPAA.

*Tristan and Isolde's story is a Romeo and Juliet type of story, with star-cross'd lovers and warring countrymen and the like. I've read the French version but I've heard that the story is originally Celtic.