You should understand one thing here.
You cannot say that it is just a game if you are playing competitively in any sport. For example, professional tennis players are registered under ATP (association tennis professionals), where they gain money for the way they play and how often they win.
The reason why sports players may be tough or violent and aggressive is because they are playing in reality for showing off their power to the world by trying to gain huge popularity and as well get a huge salary. In those circumstances players take their games really serious like it is life or death whether you win or lose.
I am right now serious. A typical player will give you a punch in the head if you tell him it is just a game. NO, IT IS NOT A GAME, IT IS A REAL GAIN OR LOSE YOU ******* JERK!!!! If you become a player any day, you will then understand how only losing leads you to nothing while you can gain something huge when you win. Winning or losing games is a big matter for players, as they can miss or succeed in their chance for winning the game to become the world champion and be known. This way it is a matter for players, where mistakes could be made, whereas it is right for them to treat themselves like it's life or death. A lose may make a big impact for them.