I think the Soviets went big for SMGs for the simple reason that they were cheap and easy to produce. Reportedly, they made SMG barrels by cutting worn barrels from M91 rifles to get rid of the worst corroded/eroded parts, accuracy not mattering much with an SMG.
FWIW, volley fire was not fire "aimed" at something like a line of enemy troops; it was aimed at an area that might not even be visible, like a railroad depot on the other side of a hill where enemy soldiers were detraining. The distance was determined by map, the aiming point would be something in the right direction that was visible, like a steeple or a prominent tree, and a whole company or regiment would fire on command. Did they hit anything? Maybe, although pretty much by accident. But the idea was not really to kill masses of the enemy, but to demoralize troops who had no idea if they would be next to be struck by an unseen bullet from an unseen enemy.