Point of detail, not "forging" but "milling" is what you mean.
The forge makes a billet larger than the final part, and you machine away all the parts that are not the final part you want.
This takes rather a lot of expensive alloy steel and reduces it to shavings., and it's not easy to police up the shavings and send it back to the smelters to make more steel (and there's the issue of the stuff is contaminated with cutting & lubricating oils (smelters are not keen on that in their mix).
So, if your steel is expensive, and your labor is cheap, and you can tolerate a generally high reject rate, then stampings are "less expensive."
And, the Soviets had experience with stamping as they were part of the PpSh (papasha), PVT-41, and PPS. Now, the parts used were designed with considerable "slop" to their fit for wartime expediency. Often the parts were designed to less-than critical to function, barrel shrouds and the like. Trunions and other receiver parts were simply shrouded by the bent sheet metal. At this "low level" of technology, only the magazines issued with the arm were like to work. It was the tolerances needed for an all-stamped 1mm receiver blank that vexed the Soviets for a decade.