Like I said - childish shit
You don’t see pro teams doing that shit - it ends in high school.
And it means a lot more when the “winner” actually wins something more than bragging rights. So in this case your “bragging” because you beat a team that moves on while you sit home the rest of the season.
I mean could you imagine a soccer team going nuts - planting their flag on the oppositions field after scoring a last minute goal to win the game 1-0 but watching the opponent move on in the tourney based on aggregate goals scored ? No - it wouldn’t happen because “winning” is more than winning a single game. They got bragging rights - we have a chance at a title.