I just find the action really fun. It's also quite addictive. I know you like fantasy RPGs and the like so it's not your cup of tea.
I do play FPS every once in awhile, but you're absolutely right. I've got Killzone 2, but I've only played that once. If I do play a first-person game, it's typically Metroid Prime or Bioshock. Otherwise, I play RPGs, action, or fighters, mostly of the Japanese persuasion.