Because she would still be relatively better at any other position so the team could be better overall.
I'm not sure how long you have been in softball, or what age your girl plays,but one day you will see a lefty at short. Your first thought is going to be, this should be good, we are going to kill them. At the end of the game you will be shaking your head wondering how you were unable to get the ball by her, and how come you never were able to take advantage of this obvious error in tactics.
The first truth of the matter is that the lefty has obvious disadvantages on the play deep in the hole, and has to back hand things up the middle. In the standard game I see at most 1 play deep in the hole that a right hander makes and even then the throw is usually too late to get the runner. As for the up the middle play, if we expect the right hander to be able to make this play in the hole, the lefty should be able to make it up the middle. Which actually turns her body just a bit to get that lead shoulder facing the direction of throw, so in some ways that backhand play benefits her.
The second truth of the matter is, for every play deep in the hole that the lefty probably can't make (but neither do most right handers) there are 10 routine plays (grounders and balls in the air), plus 5 relays from the outfield, and 5 throw downs from the catcher. If I have a girl who can make those 20 plays better then a girl who is physically able to make the once in a while play, I'm going with the girl who is better. I don't care what hand she uses, if she is the best at the most important fielding position, then she is playing it for me. Most SS are also better at outfield then most outfielders does it make sense to weaken the most critical position in order to strengthen a different one?