Consider this scenario, you get matched up against a team and are winning and it looks like this win is in the bag. This in turn frustrates someone on the enemy team who leaves. A few seconds later backfill kicks in for the enemy team and they get a new player, a really good player. Suddenly your win that you would have gotten if the leaver had stayed or not been replaced slips throw your fingers and now you’ve lost the match.
Not only that, but this could lead to people trying to pressure their teamates to quit if the team believes them to be the weakest link in hopes of a “better” replacement.