Perfect security is infinitely expensive.

Security is not some fixed quantity, it is tied to the threat model; how much the adversary invests in defeating the system's security.

Systems are embedded in larger systems; even if an inner system is "perfect", an adversary can attack in the larger system.

For example, someone physically abducting you and physically threatening you to compel you to put in your password.

But just because perfect security is infinitely expensive doesn't mean we shouldn't improve security where we can.

Incremental investments in security creates incremental benefits.

Also, the threats don't stay still, they coevolve with the underlying system.

Very good security yesterday might be insufficient security today if the threats evolve.