Why can't Air China choose the safe exit for online check-in?

Many people like to be the location of the exit, because it is spacious, but online check-in can't choose this location. You must register at the airport. The check-in personnel will arrange the location of the safety exit for you only after seeing that you are healthy and convenient for running. Moreover, in order to maintain the dynamic balance of the aircraft, the positions of the front and rear rows are often not selectable.