Because snipers carry extremely high powered weapons with a slow rate of fire. In real life snipers stand no chance quickscoping, because guns are A LOT heavier than you expect. When I first held a real 22. calibre bold action rifle, it was a lot heavier than i expected, and it contains only one TINY bullet. Now imagine how much heavier military grade sniper rifles would be- you cant quickscope. And even if you could quickscope in real life, the recoil for each shot would prevent you from firing 2 shots in less than 3 seconds. And if you "quickscope" someone in real life with a sniper rifle, there is no guarantee he will die in the first shot- he will probably kill you while you are trying to fire the second shot. And to give you a feel of how long "3 seconds" is count 1 Mississippi 2 Mississippi 3 Mississippi, and you will understand what I'm saying by high powered rifle
The ONLY reason you can quickscope in FPS games is because people die in a set amount of bullets in videogames. In real life, they could die in 1,2 or even 3 hits. There is no SET guarantee. And in the time you are firing the next shot, they WILL kill you.
You are firing digital representations of guns that are nothing like their real life counterparts. They have stats and numbers assigned to them for purposes of game balance. The developers of this game decided that quickscoping was stupid, but regenerating heath was not, despite both being unrealistic. This is a video game, for your entertainment only.