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

The Air Force was never intended to be a ground fighting unit. Back in the day, AF bases were protected by Army units for that very same reason. The ground fighting component of the AF is SF. Among our many duties, being the infantry of the AF is one of them. I started my military career in the Army and I will tell you that the AF as a whole was never designed to operate like the Army or Marines. The culture is completely different, and for good reason. The AF could not operate under the same culture as the Army or Marines. In SF, weapons are a tool for us. For the majority of the AF, a weapon is not a tool that they use in their day to day duties. ANd frankly, it should not be. I used to be CATM, and I can tell you that keeping an M-4 away from most AF members is a good thing.

Completely in jest. I have seen people in all branches who should be permanently downloaded (having their weapons privileges revoked).

Everyone sees things through their own experiences. If I were an AF Security Forces officer, I might share your opinion. As a pilot, I was required to qualify annually on my assigned personal weapon, the M-9, because aircrew members deploy more than other AF personnel. So far that makes sense. So why not go to the much bigger effort and expense of making us like the Army or Marine Corps and force every Airman to qualify annually? To put it as simply as possible, because there is a fixed amount of time and money, and there isn't an Air Force base in Afghanistan or Iraq or anywhere else we fight. When deployed, we generally either to deploy to large Airbases in-country with our Army, Navy, and Marine brethren (who are responsible to protect the base) or out-of-country where the threat is much lower (SA, Qatar, Bahrain, etc). I know there are exceptions, especially for our Special Forces community...but then they are qualified on their personal weapons, aren't they? In order to be the best at Our Job as Airmen, we spend our time and money on training aircrew, mechanics, and all the other support functions necessary to keep airpower supporting the Joint Force Commander. If we spent lots of time and money to be like the Army and the Marines, we would be better infantrymen but worse airmen.