What I'd rather ask is why baka gaijin developers haven't bothered to make a science fantasy action MMO where you get to run through instances, fight cool bosses at the end, get rare items and have tons of customization.

I mean it's Amalur's action gameplay + Destiny's visual feel + Dungeons & Dragons Online's instancing and itemization + Saint's Row's customization + get Andrew Sega or Frank Klepacki to do the music. Not like it's not done before, just not in that specific combination :O