This is something I never understood. With a book, you can't focus on background things, as it'd distract from the story, leaving readers to be jarred. However with a movie, you see things, just simple things in the background, you probably won't notice them until you've watched it several more times. Like for example, when Luke first arrives in Mos Eisley, can you imagine a book going to length to describe what was going on? No, it's ridiculous as it'd distract from the plot too much, if it does distract like that, then it's a **** book.
However, in the movie, you see it all, all these things going on, yet the story still happens, yet for that brief second, you glimpsed the magical things that are placed everywhere. Subtle, simple, yet there. Movies > Books for things like that. You can't subtle describe a scene completely in a book, hence why descriptions are brief and vague when it comes to books, unless it's describing something that is either small, simple or easily describable.
I guess it's because people take those small tidbits of information and create their own visions of them, probably why it was for the best not to do any of the books, people tend to get disappointed when things don't equal what they pictured in their mind, I remember the comics dealing with the stories in the books got a lot of flak because of it