Yes, they seldomly are, but there are many instances in which they would be if people would be willing to make more 10-hour movies (they´re not) or to make a really good miniseries instead. Shogun being an excellent example - perfect book, perfect miniseries, much inferior movie cut. More high-quality miniseries and series for book adaptions! There are so many good books made for the screen a try is often, though not always, worth it.
Of course there is the, IMHO, much worse problem with time and money and brain cells being wasted on stuff like Twilight ...