Because. according to the Laws of Football, if a player is within playing distance of the ball (generally assumed to be about two yards), then he is deemed to be playing the ball - even if he is shielding it, preventing it from going out. There's a FIFA document that specifically covers this...
While I agree about diving and time wasting, I profoundly disagree with those who say shielding the ball out is obstruction. I love seeing a skillful defender doing it - I used to be a full back myself and got a real thrill out of blocking off an attacker. I'm all for enterprising play but...