For much the same reason Phil Jackson doesn't do it with Kobe or Pop doesn't do it with Duncan, until late in the fourth quarter of close games. You don't exhaust your offensive superstar by making him play the best players on the opposing team for the full 48 minutes. You don't want them to exert more energy than they have to on defense and you don't want them getting into foul trouble. It's the reason why the Lakers have acquired guys like Ariza and Artest so Kobe doesn't have to do it. It's the reason Pop will often put Duncan on the weaker front court player on the opposing team to start the game.
I think you're getting confused with LeBron's ability to defend and his need to defend. His team is pretty good, you know. He doesn't have to play shut-down defender on most nights. But when he does, he will, especially late in games. I'm not one claiming LeBron is a top 3 perimeter defender. I wouldn't go that far. But, he's certainly not a mediocre or average perimeter defender, as has been suggested by some in this thread.
Let's put it to you this way. Say you're the coach of the Cavaliers. They're up by 1 point against a good team with a great offensive perimeter player with 15 seconds left in the game. Whoever it is against, Kobe, Wade, Joe Johnson, Brandon Roy, Carmelo, Paul Pierce... whoever. You're the head coach. Again, 15 seconds left and the Cavs up by 1 point. Are you going to put LeBron on those guys or Delonte West?
Exactly.