They don't have the correct structure of a nail. Mainly both ends of the nail. The head of the nail isn't flat: it's thicker and curves around. Like a coin. And I guess the point of the nail isn't nearly so needle-like.
You sure do know some about nailing. Would you care to nail me later? I mean, teach me your art of the screw? ;3 If Fluffs would let me, of course.