You are wrong. US was first to equip semi-auto rifles because they managed to make a good model first. There was a huge issue of poor reliability and need for better and more complicated manufacturing, that Soviets only managed to over-come in SVT-40 model in 1940, Germans in G43. Most semi auto rifle (at least for Soviets and Germans) designs rejected in 30s were rejected due to poor reliability of weapon delivered, and need to spend more time on refinements.
It wasn`t just an issue of throwing money at a problem. Making a model that was reliable, didn`t cost a fortune and could be manufactured in high volumes was a huge technical issue.
Soviet PPSh units were created due to industrial limitations, since SMGs were not just cheap to produce, they could be produced on less advanced factory equipment, with less qualified labor, which was a huge factor, especially for USSR`s less qualified on average labor force.
How is it inadequate, if more or less all that it did, was creating somewhat more capable infantry?