Neglect. No respect for the gun and not bothering to put some kind of rust preventative on at all, in many cases. Then there is the issue of coastal areas or professional carry guns. Blued guns apparently are prone to rust near the ocean when carried, and demand constant attention, and still might be a little rusty. Sweaty people's CCW guns can tend to rust without constant attention. Police etc. carry constantly and are out in the rain, weather and get exposed to the elements. So there might be some excuse for traces of rust in those cases. But I still think the no. 1 problem is neglect, therefore I would examine any gun carefully that has notable rust outside. Maybe not lubed inside either. Maybe not cleaned or alot of internal wear from same neglect. If you keep your gun cleaned and protected for carry, you will experience finish wear over a long time but probably not rust!
Your gun is like your nose, it is just wrong for someone else to pick it for you!