Why do the caps fall off the acorns?

The reason the caps are not attached to the acorn after harvest is due to the acorn growing and "shedding" the cap. An acorn is a living organism. While the acorn ages, the cap loosens and separates from the nut.

Acorns turn from green to various shades of brown as they mature. Generally, acorns are mature when they naturally start to fall from the tree, or fall when the tree limbs are shaken. The caps of mature acorns will come off.