1) There is no competitiveness between the two first teams, so to give the other team a sporting chance you pull your first stringers out to give play time to the second string, if they are still too much you go to your third string.
2) Second and third string teams work very hard, so they get some time to play, and the game "should" be more competitive at that point.
3) Baseball/softball are the only sports that don't allow a player back in, so if the other team bridges the gap in the sports where you can bring the first string back in there is no risk of giving the game away.
4) Second and third string players do have a chance to play at some point if a first string player gets injured, so it doesn't hurt to get them some field time in case they should be needed as fill ins.
5) No one wants a game so lopsided that people get embarrassed, it doesn't really do much for the winning team's confidence, and it invites a more powerful team to do the same thing and humiliate yours if the tables ever get turned, so it's just best to follow the unwritten rules.