It's just that if sniper rifles are that powerful and easy to use by anyone at any distance, standing or prone, everybody wants one.
Sure... everyone wants one.
Until they get into a proper firefight where that machineguns or assault rifles massive ROF compared to the Sniper Rifle makes a world of difference.
Your sniper rifle relies almost entirely on your skill rating.
But you get yourself an M60 and start spraying and you get a massive boost to that skill rating because of the higher ROF giving a much higher chance of getting more successes (assuming you hit), and even if you don't hit, you get a lot more suppression.
I suggest you just run a test scenario where you set up 4 players with C skills using M40A3's against 4 Enemies with C skills using M60's and make them all fire at maximum ROF and see who wins that scenario the most times.
EDIT: Oh and since it might come up... the longer range only helps if you're successful at discovering the enemy before they discover you. So that's highly situational.