The main reason? This isn't SNIPER ELITE... If all you want to do is snipe from 2 miles away, play a sniper game. From 300-400m or more, the enemy doesn't even hear the unsuppressed shot, so...

1. Why do you need to be further away?
2. That would defeat the purpose of having other rifles if you can engage from such a distance that the AI will never find you.
3. That would destroy the balance of Handling vs Range vs Power vs Rate of Fire, etc.

Seriously, Battlefield 4 has a 20x and 40x optic, both of which are totally unnecessary for the ranges of those maps if you play with 1 or 2 hit kills. This game allows targets out of combat to be killed by a single Assault Rifle round, you really don't need to be that precise, or fire from that far away. Most of my shots are with the M4A1 and they are body shots because I don't need to aim for the head. It's not worth the trouble or the chance to miss.