Keep in mind that typical movies in a theatre run at 24fps
Yes, but movies have motion blur, due to the exposure time for each frame, which makes them look more fluid than they really are. It's an optical illusion. Check out the combat scenes in Saving Private Ryan or Gladiator. They used high speed shutters for those scenes, resulting in sharper images with less motion blur. You can clearly see a slight stutter in those scenes. Computer games, including FSX, usually don't have motion blur (and even if the have it's just some filter effect and not real motion blur). Each of their frames is perfectly crisp and that's why we need a lot more frames per second to have a perfectly smooth impression in games. Sure, 24 or 30 fps work reasonably well and a frame rate cap is necessary in FSX to prevent hickups, but that's nothing compared to the smoothness of a game running at 60fps. It makes a world of a difference and it's unfortunate that we can't have framerates like that in FSX.