(Im no Developer but I can still kinda answer your question)
The more players in a race, the higher the reward at the End.
If we would stop racebanning and if the reward would stay the same no matter how many players have left, then players could easily grind Money and RP in a short amount of time.
Just imagine, 4 people start a race, 1 stays and the other 3 leave it. The 1 guy can easily win the race (the race will count as won then) and get max amount of Money and RP.
In that time the other 3 guys were able to start their own race in a matter of seconds.
The more players in a race, the higher the reward at the End.
If we would stop racebanning and if the reward would stay the same no matter how many players have left, then players could easily grind Money and RP in a short amount of time.
Just imagine, 4 people start a race, 1 stays and the other 3 leave it. The 1 guy can easily win the race (the race will count as won then) and get max amount of Money and RP.
In that time the other 3 guys were able to start their own race in a matter of seconds.