Unfortunately with up to a few dozen dogs needing to go outside multiple times a day and a half dozen volunteers that are only on shifts for a few hours at a time, there were just too many different people needing to go through that door too often for it to be worth locking it and distributing keys to everyone. Maybe having the door locked and the key hanging right next to it would be enough for people to disengage autopilot, but I'm not convinced that non-volunteers wouldn't just grab the key and open the door sometimes as well. Plus, some of the dogs could be quite strong, and the volunteers ranged from teenagers to retirees, so having both hands on the leash and being able to just push the door open was preferable in a lot of circumstances.