Why is Defense so Hard?

The basic premises is that you defend every attack vector. While the attacker probes for the one weak point where your defenses can be bypassed or breached.

Securing a computer system has traditionally been a battle of wits: the penetrator tries to find the holes, and the designer tries to close them.
— M. Gosser

When possible in the physical world fortifications are used to limit the attack vectors and defense in depth is obtained by layering using obstacles such as walls, cliffs, moats, and rivers.

A more modern description could be a football game The goal, pun intended, is to breach the other teams defense and score.

No one gets points added to their score for the number of plays that are successfully defended against.