Because they are....... If it can cut through 3 inches of battleship door, I am pretty sure that getting hit by one is going to do more than hurt. The only time that somone gets hit by one in the movies and lives, they lose a hand, etc... Obi wan survived because I don't think Count Dooku was trying to kill him, or he was trying to kill him slowly.
Lightsabers are the most powerful meele weapon IN THE GAME. Why make them even more better?
That's all you need to know. Want to make them instant kill? Activate Master Flurry with Master Speed and a trooper dies? Bah. I say, Bah! And you want the game to be "harder" and more like the movies where troopers pose a challenge to a Jedi? Well, why not have you struggle against a Sith trooper? You are a Jedi...but you are also mortal.
And no way you are going to have a person automatically die randomly becuase of an arrow. That may be realistic, but I'm not aiming for realism, I'm aiming for a good story. In fact, I actually prefer "Storyteller" games with just a GM giving you a character sheet and being the sole arbitary desicionmaker. No HPs, No stats, No nothing.
...And if you want to Nerf "Restart", just have the save file get deleted when you get killed, as they do in "Rouge"-style games.