It's funny, because the definition of "sniper" in the fifth edition of the Macquarie Dictionary, sniper is defined as "someone who shoots at other people from a concealed OR long-range position."
If going purely by this definition, sniping could be effectively executed in nerf, as all someone has to do to classify as a "sniper" is shoot at someone from a concealed position.
This reasoning is taking the dictionary meaning completely literally, and ignoring that the general consensus and practice of sniping is long ranged, completely useless in nerf. Also, sniping can be seen as cool by a certain demographic, and then become the "class" of choice to those people. With so many "snipers" the quantity of good game play is dramatically reduced (think of putting 18 Full Forwards on a footy field at once).
Also, nerf is not about simulating wars exactly. We don't have medics running around hauling people of the field when they are hit, or men digging trenches and setting up cable lines for communication. Why do we need snipers, they add almost nothing of any value to any nerf situation. We don't need snipers, and we don't want snipers in nerf.
Anyway, who needs to snipe when you can rush though!