Sure, then it's supposedly good for the person who leaves. But that leaver just screwed over their entire team. The game isn't decided until the nexus actually explodes, but if someone is allowed to leave, their team's morale instantly drops to rock bottom.
Overall positivity in League (which admittedly isn't great, even at the best of times) would probably plummet if teams weren't locked together until the Victory/Defeat screen. It's a team game for a reason; giving individual players more ways to impact the entire game experience for everybody else is not likely to be a good thing.
This kind of feature is tailored for those that are playing as individuals, which is not the intent of League of Legends. In the ideal scenario, we want players to be thinking "What can I do to increase my team's chances of success in this situation?" rather than "How do my stats look?"
We don't want League to turn into COD. It's a horrible role model for a game that wants to encourage a positive and team-oriented experience.