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

The fact is the USAF, just like the Navy and Coast Guard are mostly technicians. Few, with compared to the overall force need to qualify on any weapon. In the USAF, the groups that need to qualify on any firearm are the Security Police, Special Forces, and aircrew members. If deployed to a combat area the SPs are responsible for base defense (along with any Army or Marine forces that share the base). The need for SF to qualify should be obvious to anyone, and aircrew need to qualify because they might be in a survival situation behind enemy lines if they ever have to leave thier aircraft inflight or after a crash landing.
During the Vietnam War, all USAF personal had to qualify on the M-16A1 prior to deployment to Vietnam. Remember that during the Tet Offensive many air bases were attacked and Army and Marine units were tied up in thier own defensive or offensive operations.
I myself qualified on the M-16A1, M-1911A1, and the S&W .38 revolver. As an aircrew member, I needed the M-1911, and later the .38 as a survival weapon, and at one time had orders to Vietnam and qualified on the M-16 and M-1911. Those orders were later canceled, but I later deployed to Guam and after cross-training to a KC-135 Boom Operator deployed to SEA, and towards the end of my career to Desert Shield/Storm. During those years I maintained my qualification initially on the M-1911, then on the .38 every year.
Don't forget the USAF awards the small arms marksmanship ribbon to those qualified on any of the small arms used by USAF personnel. Some personnel also need to be qualified for the combat readiness medal.
https://en.wikipedia.org/wiki/Marksmanship_ribbon#U.S._Air_Force
https://en.wikipedia.org/wiki/Combat_Readiness_Medal

I served USAF from 1964 to 1984. Qualified in basic with .30 carbine, went thru Air Police (Security Police) and qualified with .38 Revolver. Cross trained to Inventory Management and for whatever reason qualified with M16 several times prior to 1977. Assigned to Red horse (Combat Engineers) 1977 to 1980. Qualified with M16, .38 Revolver, and M79 Grenade Launcher during that assignment. During Viet Nam Air Force personnel were qualified with and were initially issued M16's. In an attack on Tan Su Nut Air Base so many Air Force personnel shot each other resulting in the disarming of just about all Air Force personnel. While assigned to RAF Bentwaters, England 1965 to 1968, I was a Security Police augmentee and carried a .30 carbine during alerts guarding loaded out aircraft. During that three years one of our augmentees shot a rabbit while on post, and several times augmentees fired a shot into the clearing barrel during weapons turn in at the armory.
I totally agree that all military personnel have basic weapons training and proficiency training and if they can't behave properly and practice weapons safety they can be sent on their way

My career in the USAF saw me not in Special Operations but in Special Projects and as such I had to be able to use many weapons. I started off with the M1 Garand in JROTC. In the early 1970's Enlisted Basic Training had me learn the M-14 and M-16. My duty with Air America had me learn the AR-7 survival rifle, S&W Models 10, 12, & 15 .38 revolver, Colt M1911A1, Laotian M1917 American Enfield, and M79 Grenade Launcher. I had to re-qualify with the M16 and the S&W Models 15 .38 revolver from time to time because I was a SP augmentee. In the early 1980's Officers Training School had me re-qualify with the S&W Models 15 .38 revolver. After that I did not touch a military weapon until Desert Storm where I again had to re-qualify with the S&W Models 15 .38 revolver. I was already in country prior to Desert Shield and did not ship with a weapon so I had to take what was available. If you were going to a place where you may be placed in harms way you were given training on either the M-16 or the M-9 semi-auto and shipped with a weapon or issued one in country.

I was medically retired in 1992 so many things are different from the USAF I was a part of.

Being in the Air Force is like "Military Lite". There are more civilians (never served) who know how to shoot and maintain arms they are presently serving in the Air Force today. ( remember, this is more tongue -in-cheek than fact)....

As a new slick sleeve Airman I qualified with both rifle, M14 and pistol Colt 1911 A1 on the Lackland range in 1963. In fact I have the ribbon they awarded us for good shooting. (The Air Force had no expert medal back then. We transitioned to the .38 Colt later on.

When one considers the nature of many military operations over the past two decades, it certainly would not hurt to change the training paradigm. With the blue to green (USAF and USN) filling in for US Army in Iraq and Afghanistan, most expeditionary units that took on those deployments did do training to accomplish that mission. It woudn't hurt one bit to provide/mandate weapons. commo and combat casualty care to many AFSC's. If you're a drone operator out at Nellis AFB, who cares; but if you're going to fill in for Army or work with the Marines, absolutely it is needed.

I feel that basic proficiency with a rifle or carbine should be mandatory for all service members. However, training AF or shipboard sailors to "fully qualified riflemen" would take too much time away from other training and duties and ultimately reduce the lethality of the entire armed forces. it is hard enough to train real riflemen with the limited availability of ammunition range time.

Of course I’m old school but when I went through Lackland we qualified with both rifle and pistol. The Air Force doesn’t award a medal for marksmanship but I received a ribbon for small arms expert. Of course times may have changed since 1963.

All personnel of any branch should be familiar with the small arms at minimum that are in the armory of that facility, I was an AFSC 81152 and ABGD, we knew our stuff..

I think every person in the military should be taught basic rifle or pistol skills.