I think the real reason game developers completely ommit realistic sniping in gameplay is due to a couple of facts:
-real life sniping can be complicated, you have to adjust your scope for distance, elevation, windspeed and target speed before you can fire
-you have to go prone and set up the sniper riffle and are pretty much rooted to the same spot.
-if real life sniping was implemented, the sniper class would literally decimate any other armed units from long range. There is a good reason why snipers are referred to as force multipliers. Even one sniper can cripple an entire unit.
Also at such extreme ranges, snipers can be real pains. You have little indication where he is firing from and his shots cause a lot of damage (or in real life sims. kill you immediately). The only weapons you have against such snipers are smoke grenades, running for cover or using mirrors/bright light etc. (which is kinda silly I reckon).
Imagine in Far Cry those sniper dudes shooting at you from 800m-1km away.
If I am not mistaken Full Spectrum Warrior and Americas Army allow for a realism mode where each shot is deadly, flash bangs at close range do damage and the smoke grenades take 4 minutes to start off etc.