Some reference I found, although I don't support the conclusion.
They originally were large hammered nails used to mount the door panels to the (usual) Z-frame of the door. When using a door, the Z-frame and the panels will always develop some friction, the nails being worked loose, the door sagging and sticking in the doorframe. The solution would be to hammer the doornails senseless again, and again until they were really finished.
I think that is the origin of the phrase, simply having to whack these nails again and again to keep the door in shape until the nails finally broke off.
The reference:
This is old - at least 14th century. There's a reference to it in print in 1350:
"For but ich haue bote of mi bale I am ded as dorenail."
Shakespeare used it in King Henry VI, 1590:
CADE:
Brave thee! ay, by the best blood that ever was
broached, and beard thee too. Look on me well: I
have eat no meat these five days; yet, come thou and
thy five men, and if I do not leave you all as dead
as a doornail, I pray God I may never eat grass more.
As 'X' as 'Y' similes refer to some property and then give an example of something well-known as exhibiting that property, e.g. 'as white as snow'. Why door-nails are cited as a particular example of deadness isn't clear. Door-nails are the large-headed studs that were used in earlier times for strength and more recently as decoration. The practice was to hammer the nail through and then bend over the protruding end to secure it. This process, similar to riveting, was called clenching. This may be the source of the 'deadness', as such a nail would be unusable afterwards.
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