Because this is a game.
It doesn't matter if weapon X is more powerful than weapon Y in real life. If pistols were made worse, no one would use them. If the SMGs were more powerful, no one would use the assault rifles/pulse rifles. If the Assault Rifles/Pulse Rifles were more powerful, no one would use any either the SMGs or the Pistols.
Hell, if miniguns were accurate you wouldn't be able to carry them and firing them would rip the gun out of your hands. Even a 22.cal minigun in real life cannot be wielded by a human.

Even though . . . I "wish" the game was at least as "realistic" as Arma2 when it comes to things like small arms performance . . . Vermillion makes a lot of good points. Most of the deficiencies in the way small arms are represented relate back to the vanilla game. Attempting to "adjust" a lot of this stuff so it seemed more sensical would have repercussions throughout the entire game play loop.

Having said that . . . I have often contemplated diving into those files and just whacking every single number up or down or sideways so that it is "realistic" and then seeing how that plays The thing is: I don't have time for that and I'm lazy and retired! So I KNOW that Vermillion who is young and employed and does this out of love for the project doesn't have time to do it

One thing that prevents any degree of realism is the rate of fire and reloading limits.
That maximum rate of fire possible for a sightless weapon in-game is 600 RPM. Giving up the sights affords a maximum RoF of 1200 RPM (currently only used by miniguns).
Modern SMGs have a minimum of 800 RPM, and can go as high as 1500 RPM. The MAC-11 can deplete it's entire magazine in 2 seconds.
Even modern assault rifles fire at least 600 RPM.

Of course, none of these weapons are made from pure iron in a 3D printer running on solar power firing iron bullets out of copper casings.

I personally really like the way the firearms are differentiated, with pistols optimised for precision kills and rapid automatics being a little more flexible at the expense of juicy headshot multipliers.

I used to main the old Epic Pistol before Eleon nerfed it into oblivion.

Anyone that sees a weapon in a game and thinks "but in real life:" should not work on game balance, this isn't real life this is a sandpit made of triangles for us to play in.