Why stale bread gets hard and the wafer, soft?

The bread dough is composed primarily of wheat flour - water, yeast and salt.

The flour, in turn, presents two main components protein and starch.

 It happens that the dough before baking, the starch molecules are arranged in granular structures, in which water does not penetrate.

 Upon heating, however, this structure breaks down and the starch molecules combine with the water released by proteins coagulated by heat, forming a sort of gel. 

As the bread begins to cool, the starch molecules are reorganized, releasing water and forming a rigid structure, which causes hardening of the bread.

 Not so with the wafer for three reasons.

 First, the amount of flour that goes into its composition is much smaller.

 Furthermore, the fat cookie dough binds with the starch molecules, preventing them to reorganize and harden.

 The wafer is also soft because its mass is very dry and exposed to air, eventually absorbing moisture.