Answer:

When a refrigerator door is left open, the room becomes hot because the quantity of heat received by the refrigerator, as well as the work performed by the refrigerator, is rejected into the room.
The temperature in the room was going to rise. If you keep the door open, the heat from the room is simply recycled into the refrigerator and then back into the room. The engine heat, which would be running constantly to move energy around in a circle, would result in a net increase in room temperature.