Why do those who carry firearms for living fail to master it?

Last Friday I went to pre-qualify for an NRA course. I have never taken any NRA courses before and the instructor wanted to make sure that I am familar with safety rules and can shoot my firearm well enough to take the class. Right before me there were 6 or 8 security officers quialifying with their side arms. I was shocked at how poorly they did. I am not a crack shot by any means and in fact I consider myself to be mediocre. However, I saw my groups to be tighter than most of the ones by the security officers and I was shooting a DA revolver vs a modern semi-automatic (I saw many Glocks, XDs, and one 5 shot snubby – one of two shooters who did well). The target was a large stylized body outline (Q target), distance was about 7 yards, and speed of fire was slow to somewhat rapid. I saw many misses as well as truly poor grouping. I can only imagine what the shooting would be if they were under true stress. In addition I saw how one of the officers managed to load a .380 into his 9mm handgun and was puzzled that it had a failure to feed. The reason I was shocked as oppose to just amuzed is that not only their livelihood depends on their shooting skills, but their life! In my professional occupation I try to take skill improvement courses and attend workshops relevant to my job. I do less than I think I should – I do not read trade magazines due to the lack of time, I do not write articles, don’t go to trade shows, etc. However, this does not have an impact on me living or dying. Excellent shooting skills on the other hand have a direct impact on one’s life if that person is carrying a handgun to defend oneself “in the greatest extreme”. So why would one not spend the extra effort to be a good shot, be familiar with defensive tactics, etc?