If you're talking about cases like benjy it's because players might have insider information that the general public does not have which would give them favourable bets compared to normal people.
Scrims, mutual connections, players/coaches having private conversations with trusted close friends or even just straight up asking. You can't remove all potential sources of this kind of betting but at the very least the players and anyone directly involved in esports needs to be held to a certain standard. Of course they could just ask their friends to bet for them or whatever and I'm sure it happens but for the sake of rules anyone who could potentially have non-disclosed information is restricted from betting (to the best of their ability to enforce restrictions)
Many players would probably enjoy betting just for fun and wouldn't try to rig but you have to make rules to prevent (as much as possible) abuse and unfair profits to those who may want to rig the odds. Even moreso now with riot allowing betting sponsors in valorant they want to uphold their credibility and reputation for being fair