It takes skill because you aren't sniping in a void against CPUs. It doesn't matter if sniping in destiny was the easiest thing that has ever existed in a video game ever (which i don't agree with but suppose that it is), you have to execute this simple action FASTER and more accurately than your enemy who is also trying to do the exact same easy thing. The skill isn't really ever in the difficulty of the action, skill is always relative. So yes, sniping an npc that isn't shooting you takes no skill, but sniping someone trying to snipe you takes skill. If nothing in this game took skill then most gun fights would end in draws and everyone would have around a 1 kd.