If they wore special devices that monitored their health while playing they would be better able to understand their own health.
Feedback from fans would just include negativity which is not good for anyone. Listening to their coaches from the sidelines would distract them from the game and potentially cause injury. Using a device to sense resistance levels of other players when they tackle them would not be beneficial to the players as it is not measuring anything to do with their own health or resistance levels.
