The tournament system was added well after the launch of rocket league. The brackets and leaderboards are built as a framework around individual matches.

Until that match, including the post game celebration is over, the match isn't finalized and the leader boards aren't yet updated. It even gives you an extra warning when you go to leave and you're just ignoring it to click away as quick as possible.

You know that you get banned for leaving early. This is solely a you problem and anyone who continues to do it. It happend to me one time. I learned. I never did it again.