I am going to share some hearsay information I got from on older retired airline pilot who was also a naval aviator. From his age, he was flying from carriers in Vietnam. He flew airliners for Piedmont, then TWA and retired from American. He is my GA flying buddy and we had a very unprofessional almost clueless controller in a very rural area. I asked him the same logical question you wondered: Why not hire people with controller experience out of the service? His answer: FAA doesn’t want to have to break all their bad habits (Obviously there are different standards between military and civil controlling). And I replied “they took you with your bad habits” and his response, “yeah but I can fly a plane”. So take that from it what you will. I tend to believe every word of it and agree FAA can be a bit behind the times. Hopefully they have improved in the years since this guy retired from civil aeronautics.