Just that the MMORPG is rarely as detailed as a flightsim.
Either you have a MMORPG that is focused on multiplayer experience like WOW, which looks pretty ugly or you have a limited scale like Warhammer or you have great graphics like Age of Conan but crappy gameplay and critical FPS drops with more players interacting.
Besides that, I'd like to mention that the 700 aircraft mentioned were not engaged in one single dogfight, but in raiding dozens of targets along all southern England. So it's not necessary to plot 700 aircraft + 700 interceptors in real time.