One F-35B would make a frigate worse at its real job

Why not put a single F-35B on a frigate?

Because one fighter offers little sustained combat value and forces the frigate to sacrifice space and anti-submarine capability for the least efficient way of operating the aircraft.

The F-35B would need vertical takeoff and landing, specialist maintainers, different ordnance, and more support space than a helicopter. A single aircraft also provides no useful depth when it is unserviceable, refueling, or already committed. An embarked helicopter can hunt submarines, scout, move people, and cooperate with the rest of a task group, while fighters work better concentrated on a carrier in useful numbers.