Continuity mistake: After Fred secures the chain lock to the front door, with the chain mounted onto the frame of the door, Fred then slides the chain into the door's slide-catch to show Lucy the "burglarproof chain." When Ethel opens the door the chain itself is ripped off the frame, leaving the chain (and the piece that was screwed to the frame) dangling from the slide-catch attached to the door, but in Ethel's closeup the chain is still mounted to the door's frame. (00:01:40)
Answer: There is no reason. It's a just a silly plot device, typical of the era. Women characters were often portrayed as making uninformed assumptions or decisions.
raywest ★