The thing about ALL weapons having the same mods is that, largely, many would become Waaaay to good.
I mean think about it, if you could put a silencer on a Hunting Rifle or Sniper Rifle, you could almost always get one-hit head shots and nobody would even know where to look for you! If every weapon could get a laser sight, most weapons would have little to no spread: this would overpower the minigun, Light MG, and just about every other automatic weapon. And don't even get me started on Extended Capacity Grenade MG *shivers with fear*! So it makes sense for weapons like the Varmint rifle to have a night-time scope, a suppressor, and extended mags, but doing the same thing on every single weapon (in many cases) would OP them or it simply wouldn't make sense.
Ofcourse not every weapon should have every mod. I mentioned making the service rifle fire full auto due to the fact that it's a little underpowered, another thing that could be done is limiting the number of upgrades to 3.
Heres a senario, a DLC comes out that gives the Assault Carbine 3 more upgrades so there would be a total of 4 mods: A reddot sight (would be so awesome), a silencer and a precision barrel (less spread) and extented mags. I pick the precision barrel and the reddot sight, then I would have to make a choice, do i want a silencer or extented mags? that would prehapes keep weapons from being overpowered.
Another thing is even if some upgrades would make weapons overpowered... Well, it is a single player game, I could just choose not to use them