Why don't all members of the Air Force have to be fully qualified to be a rifleman in case of hostile events?

It is because of the Korean War. Only infantry had weapons training. Cooks, Admin personnel and any other MOS were not required to be range quailified

I too believe that all members of the forces, with the exception of medical personnel (non combatants) should at least know how to fire the M16 or what it is now days they are using (M16 when I was in Security Forces also) and then if the medical person wants to be qualified, then they ask to be. But if not mistaken, if you are in medical you work under the red cross.

Good question and my short answer is that is it is not an effective use of resources. When I first went in, in the early 90s, we had our basic training instruction on the M-16. After that it was may once a year then every 3 years. Since I was in medical I was considered a non-combatant anyway. As much as I loved firing and had my own personal guns, I didn't really see the point in the Air Force only having us go every three years. Now if I was in Security Forces, I would want some range time at least monthly. I mean either justify it and have us get really trained or make us familiar with the weapon in case of an extreme situation. That is pretty much the route AF leadership went. I think our regular jobs were demanding enough and some are extremely technical, but I would have given the effort to be proficient if ordered to. I just never was. Oh and I am not saying the Army doesn't have some extremely difficult MOS's either.

In the Coast Guard, if you are on a Shore unit, you qualify regularly, or supposed to. Usually at the Nearest Army Base, When I was at Cape Cod Canal, it was Ft. Devons. Until I got Deployed to Vietnam, it was with the Colt Model 1911 .45 and M1 Garand. On the way to Nam, I was introduced to the Mattel special ( M16 ). Although the CG is NOT under the DoD, it does maintain a military prescience. Aboard ship you are constantly running drills and once a year you get to spend two weeks in sunny GITMO and run through the wringer by the Navy. Hopefully you get the big "E" across the board. Shore units ( Lifeboat Stations and such ) they usually run something called "Operation Webfoot". The goal of the aggressors is to sabotage the station. Our goal was to make sure they didn't. Not a good feeling to go down to the boats in the morning and find a mock up up a bundle of Dynamite and a note that says, "BOOM !!!!". I have heard that they used Navy Seals for that, but not real sure of that. I am of a mind that they were just Navy Divers, but possibly CG divers for that matter. Web Foot, GITMO, Longest two weeks of my life. In Vietnam, we operated under ComWesPac 7th Fleet.

As an enlisted military policeman (1950 to (1958) I can agree with the argument that every airman should qualify with a rifle at lease once a year. Later (1967), as an officer in Vietnam, I was shot at - but was not allowed a weapon to return fire. That was sad. Fortunately, the army and marines drove the Viet Cong off the base. Now, I believe that every American citizen should qualify with a rifle. (provided they are physically and mentally able). Atomic bombs are for a different war than a take-over of American cities such as Washington D.C. experienced on Jan 6, 2021. If they were qualified with a weapon, their attitude gun ownership would also change.

If you're crew and lose your aircraft (mechanical, enemy fire), it still is necessary to be competent in handling combat arms to find your way to safety

Meant to put plane not plan.

If your plan goes down you need to be able to defend yourself and UR crew

I was a CE IMA FOR 25YEARS(8AD)FOR THE MAJORITY OF MY CAREER I MAINTAINED MY M9 M16 QUALIFICATION ONE YEAR WHILE ASSIGNED TO LANGLEY WE WENT TO THE CG RANGE WHERE A FEMALE AIRMAN PURPOSELY FAILED QUALIFICATION THE FOLLOWING DAY AS I ENTERED THE BASE WHO DO YOU THINK I SAW ON THE GATE?WHAT A DANGEROUS JOKE

This reminds me of the clerk/typist/translator in Saving Private Ryan. Although it was Army personnel.
Capt Miller: DId you fire the weapon in basic training?
Cpl Upum: Yes Sir
Capt Miller: Then grab your gear ......