From the point of view of a developer, fantasy is relatively easy, and that alone is probably one of the major reasons for its success. You can pretty much get away with anything as long as you follow some basic rules. That's especially true when you're dealing with high fantasy, the most popular genre for CRPGs. For the uninitiated (like me - I just figured this out yesterday), high fantasy is that particular blend of fantasy that has no relation whatsoever to the Earth we live on. It stands opposed to low fantasy, which usually starts on Earth and then flies off into the realms of high fantasy. With high fantasy, you are free to make up your own rules and world, and I think that in general, audiences have an easy time believing in such worlds, provided certain conditions are met of course.

It's probably all to do with that dreaded and over-abused "suspension of disbelief" concept. In high fantasy, the only thing an audience needs to accept is that there exists indeed another world, and that's basically it. Once they've done that, the disbelief is easily suspended and there's nothing to remind them of the fact that it's all not true and that they are not really superheroes.