Because they were making tens of thousands of SMGs a day. They quite literally had entire armies equipped entirely with SMGs by '43. That isn't hyperbole; they'd re-equip multiple corps uniformly at the same time with the expectation that they'd be advancing continuously and almost exclusively experiencing close quarter combat in urban areas.
That rack of gats, fully loaded magazines and all, probably cost considerably less than a bomb load whilst still being able to shred convoys. If it works, why not?