In earlier times, before mass production and the assembly line, when things were made by hand, the hardware of a house was one of the most valuable parts. A log cabin might be thrown up fast to shelter for a winter to satifsy the homesteading rules, but as soon as the landowner could afford, a frame house was built. The logs of the house had little value for reuse and so, often, the log house was burned so that the ashes could be sifted for the hardware. The hinges and hooks and nails could be used over in another house or shed or barn.
Doors were serious things, needing to be strong for protection, if not literally to protect against invaders, at least to make one feel safe in case imagined horrors came to be. A common way of making a door was to lay boards the long way to the doors dimensions for the outside, then cross them with another layer for the inside of the door. This made a solid sturdy door two layers thick. The layers were nailed and the nail ends bent over on the inside. This left the layers of the door free to flex against each other in changes of humidty and temperature with the seasons and as the inside was heated in the winter. Because these nails were bent and because it took as much work for the blacksmith to straighten them as it did to make new, they were not reused. So doornails were dead. Dead as a doornail.