Because they need an excuse for humans to be special. I think there's a bizarre tendency among genre writers to justify the real world when writing their speculative ones. Which is how sci-fi and fantasy get their unintended fascist tendencies.
I used to be a fan of the Drizzt books, for example. And there was this whole running thing in the Drizzt journal entries during the Time of Troubles events, where Drizzt tried painting having "faith" and never meeting your god as inherently superior to how the gods of FR were prancing around at the time, having tea parties with their followers. Made funnier by the fact that Drizzt had met his god, but then started dancing around it by claiming it wasn't his real god, he just imagined it, so his faith could be pure.
Then there's also the standard trope in fantasy, that I think Mass Effect 1 used, where humans have comparatively shorter lifespans than the species surrounding them, so this means that humans are eager to prove themselves and live life to the fullest! Most science-fiction/fantasy seem to either paint humanity as the "eager/innovative" race or as the jack-of-all-trades race. Mass Effect tried doing both. I actually find it kind of disturbing when you think about it, because the bottlenecks that aliens, elves, and dwarves are faced with imply either cataclysm or eugenics.