Not to offend, but have you ever heard of a bighorn or elk kill beyond 2000m? While you can certainly use a hunting rifle to kill at some fairly significant ranges - and you absolutely can use the same cartridges to fire out to even more extreme ranges - sniper rifles tend to be made to a finer tolerance than the hunting rifle you got down at Bass Pro, or to both finer tolerances and more extreme use. A weapon is more than just the rifle, as well; it's the entire unity. Yes, you can get civilian weapons that will fire out that far, and in all actuality most of them probably do - but the accuracy curve goes down further. Can you refine a civilian rifle to get those tolerances? Sure. And y'know, that's what those modifications in Arsenal are for.
When it comes down to it, it's a game design issue. Yes, you can just say 'oh, all rifles are X', but when it's blatantly clear that a German cop or a US Marine are not carrying around a .30-06 they got at Outdoor world, you need to differentiate - in the weapon class, the damage done, the availability, etc.
Game design tries to approximate real life, but that's mostly what it tries to do - approximate. You can refine one part or another, if you like, for your own game, but for design issues including but not limited to playability, things are probably best as they've been portrayed for the last 4 iterations.