Every competitive game does this, and it's purely a "competition" problem.

Let's imagine: The player quitting your game is bronze, the joining one is Silver. Now, not only teams are not balanced anymore, but the player that just joined is playing on an alredy started (and maybe almost finished game) grating him MMR (or points in OW), this means that if you're about to lose and someone quits, and you join in instead of him, you just lost points without even playing a match.

If you instead don't give points for an alredy started game, every player that gets in one will just quit because there is no reason for them to stay and play.

Every competitive game does this, and it's purely a "competition" problem.

Let's imagine: The player quitting your game is bronze, the joining one is Silver. Now, not only teams are not balanced anymore, but the player that just joined is playing on an alredy started (and maybe almost finished game) granting him MMR (or points in OW), this means that if you're about to lose and someone quits and you join in instead of him, you just lost points without even playing a match.

If you instead don't give points for an already started game, every player that gets in one will just quit, because there is no reason for them to stay and play.