Ball change

Question
Why does a major league ump toss out a ball if the pitcher throws it in the dirt but they don't toss out a ball that has been hit and hits the ground, why does the ball stay in play? It seems to me that the ball would receive more damage by the ground off the bat.

Answer
That's a very good question that I have often wandered myself.  There's really no right answer except that that is the Major League Baseball Protocall.  Obviously the objective is to insure that every pitch is made with an identical baseball, explaining why they throw out balls that go in the dirt.  However, I'm just like you and have never understood why they don't do that after a ball is hit in play also.  The only reason I can think of would be that it's a very quick process to change out a ball after a pitch since the catcher is right there.  It would tend to slow down the game more if balls from the pitcher were constantly being thrown in for a new ball.  I'm sorry I don't have a definitive answer here, but then again I'm not sure there is one.