In German the translation for "fantasy" and "imagination" is the same word (and maybe they can be used as synonyms in English, too, on certain occasions?), but for me the fantasy genre is often antonymic to "imagination", because most of the time I get the feeling that I've heard it all before. But maybe that's part of what it's actually about and I just don't get. I mean, fairy tales and mythology also have reoccurring structures and repetitive tropes. And like others said, I think authors don't just use these tropes out of laziness but because a lot of people seem to love and expect them, and maybe because the authors themselves enjoy them, too. Other genres like whodunit or thrillers have pretty rigid structures and tropes as well and are also incredibly popular.
Still, personally I don't really like the fantasy genre's emphasis on epic tales that seem to be more about worlds and wars and gods and races etc. and less about personal stories and mystery. (But I still put up with them out of boredom. ;) And I find it easier to bear in games than in books, for some reason.)
Anyway, I agree with Titanium. Apart from fantasy being a popular genre that doesn't involve too much risks, I think the combat system is also a huge factor (it's certainly no coincidence and not just due to the changing times that the gameplay of Mass Effect is more akin to a shooter than a turn-based RPG for oldschool gamers). Still, that's no excuse for not trying to create original worlds and stories that feature medieval weapons. :P