First, there are air defenses near sensitive areas. Second, there are procedures for identifying lost aircraft and aircraft with malicious intent. In fact, there are...on any given day...20+ aircraft that violate the "Temporary Flight Restriction" airspace. Third, even with a lot of training, it is not easy for a shoulder fired missile to find a moving target that is as small as a Cessna or even a corporate jet. A missed shot will probably land in a densely populated neighborhood. The odds of success versus the risk do not warrant the use of shoulder fired ground to air missiles.