Why fewer than eighteen fighters cannot sustain fleet air defense

What is the logic of aircraft carriers carrying fewer than eighteen fighters if eighteen is the minimum for air defense?

They provide on-call cover or strike support rather than a properly sustained combat air patrol.

The distinction is about persistence rather than whether a fighter can launch at all. A small air group can answer a specific alert, escort a strike, or provide brief local cover. Sustained combat air patrol requires aircraft airborne, replacements ready, and others undergoing maintenance or crew turnaround. Below the cited threshold, the carrier is not functioning as a durable fleet air-defense system.