Because it is impossible to enforce fairly, or what people will believe is fair, and it WILL become a defensive strategy. Every time you have a slapper take a couple steps toward 1B on a third strike, the catcher is going to wing the ball toward first and before the umpire can raise his/her hands to kill the play, the defense will be begging for an out and cry vociferously if they do not get it.
That's the kind of answer I was looking for. I'm not sure I am convinced that umpires couldn't make fair calls on these type plays, but I respect that argument, and it helps me make sense of the fact that strikeout victims are allowed to make plays for their teams after they are out. Good point about a runner put out at second.