It's because the interface console itself is a big part of the cost and complexity of the thing. The simulator requires the console and input manipulators to use, so they can't train on it while it's being used for surgury. Hospitals could buy a second console to use exclusively with the simulator but that's a multi hundred thousand dollar piece of equipment.

Surgeons can take a few practice simulations in the Sim, but it's just that doing so ties up the robot from doing real cases at that time also.