Well in the real world guns don't even seem to hit people all that often. So I am just basically guessing that big heavy weapons designed to kill 10+ ton objects due to their size, weight, and normally lower firing rate would have even more issues in hitting.
Or take it this way: If on a firing range novice shooters in the Navy have to hit around 12 out of 20 shots on an immobile target with a pistol to pass that bit of basic training and I was a GOOD shot and got like 17 or so with a .22, on a battlefield with everyone running around and high stress cuz things can kill you and the chaos of war and cover and such big nasty weapons are going to be difficult to hit some dude.