Well... I have 2 references to that...
1) I was in the Army. At best, we shot qualification once a year. Once we got past Basic, there was no additional instruction or training on weapons marksmanship. Shooting (and handling firearms) is a perishable skill, and without recurring training... well, you get the idea.
2) Way back in the early '90's, a local gun shop advertised retired police revolvers (4" S&W 686's from the Westerville, OH PD...) I went in to check them out... the salesman pulled this big box out from under the counter... all the pistols had been, basically, dumped in this box. Anyway, he laid about 8 of them out on the counter... of those 7 of them looked like they had never been cleaned... ever. Dirt, cruft, damage, fouling. I was lucky enough to pick the diamond in the pile... a piece that had obviously been well taken care of, it was clean, polished, and I think the trigger had had some work on it. Those other 7 pistols, and I'm sure most of the others in the box, got me to thinking about service pistols... just because you carry a pistol in execution of your duty, doesn't necessarily make you a firearm enthusiast. Just like some of my brothers and sisters in the Army back then could care less about the M16's they were issued, let alone their skill with them, carrying a service arm is likely thought of as a nuisance rather than an advantage, and the idea of training with it a waste of time.
I agree... a trained operator is certainly more desirable than a generic manual safety, because at the end of the day the only real safety is a competent operator. But, living in Realville, and knowing many service weapons are carried by people with minimal training and/or poor attention to basic competence, a positive manual safety is a reasonable solution.