The reason fantasy (and also much scifi these days) needs the length is partly because once you've gone to all that effort of establishing a world, you'll want

The reason fantasy (and also much scifi these days) needs the length is partly because once you've gone to all that effort of establishing a world, you'll want to stick that world. A short novel with a completely unique setting would turn a fantasy story into something closer to a fairy tale or fable, or be entirely about the setting (as is often the case with scifi; Lem's "Solaris" is a great example). Of course, you can still tell completely separate stories set within a single world. Which authors do: Banks with the Culture books, for example, and I rather prefer that form of non-serial storytelling. G. R. R. Martin himself has written spinoffs set in the Westeros world. Gene Wolfe's novels are often set in the same universe. The second aspect of serial, "long-form" storytelling that everyone enjoys is the continuity; you may have lots of small stories told over the course of several seasons of a TV series, but it's the same, familiar world, and you don't need to be introduced to a completely new setting every time. As someone who greatly enjoys books (though not so much fantasy) but has a huge problem getting past the opening chapter or even the opening page, I completely understand the attraction. (That said, there's no excuse for bloat. G. R. R. Martin seems to have entered into a kind of narrative tailspin as a result of having plotted himself into several corners. Rather than chopping away at the narrative and making some shortcuts to get the characters in the position he had planned for them to be in, he apparently decided the solution was... more plot. The end product was the long and tedious boat journey that was "A Dance With Dragons".)