Why?
These airplanes are guarded by people carrying loaded guns, which is pretty secure.
You don’t want keypads or cardswipes for combat airplanes because that can slow people down and sometimes you have to get these things up and running as fast as possible.
In any case - even if someone DID steal one, unless they knew what they were doing they’d probably kill themselves on take-off.
I can’t speak to USAF aircraft, however, US Army helicopters have keys. They look like any other key.
I don’t know how things are being done today, but back when I was in the Army there were chains welded to the vehicle and locked around the steering wheel to prevent unauthorized personnel from taking off in them.
My husband is a firefighter for a major aircraft manufacturer. For several months after 9/11 the fire dept had their big trucks (so big, they can’t go out on the street without an escort) parked across the flight line so if someone tried to take an airplane, they’d run into the fire truck.
Aside from whatever controls inside the cockpit you need to start a fighter, what about external equipment?
Obviously, there’d be variation between different models, but I recall that an accident with an aircraft starter unit onboard the USS Enterprise caused an explosion and fire in 1969; and that the engines of the SR-71s could be started either with compressed air, or a large piston engine mounted in a “start cart” that could be used like a starter motor.
Not intending a hijack, but a lot of construction equipment rental places do similar on a daily basis - at the end of the day, they’ll park anything with a “good” ignition key* to block the gates so anyone bent on stealing a backhoe or anything that travels on a trailer has to get through the fence and move a truck.