Because it's financial fraud, and it harms the legitimacy of the market itself and it harms the other people participating in the market. It's not like banning a cheater in a video game, there are real world stakes.
If we are playing poker, and you cheat, you have stolen my money through fraud.
Certainly if I cheat at a casino, I'm likely going to jail.
Also, perhaps there is a differentiation between working within the mechanics of a system to cheat, and going around a system to cheat. An example from the esports league would be the difference between using a corner case to shoot through a wall, versus hacking into the server and modifying the code.