You can't view both sides of the train at the same time to perform the train safety check before pressing the close doors button.
That's why the doors are fitted with "hustle alarms" and are final close pressure sensitive as well as being interlock fitted. It used to be managed without problem at Finsbury Park many times a day with loco hauled slam door stock as well as DMU slam door stock. When the 313s appeared in 1976 - or was it 7? - it was like seventh heaven for guards.
Still, time and sueing culture moves on........