They aren’t. Originally NFA was going to apply to ALL firearms with a short barrel. The plan was to make handguns available only to the rich and the government. But it got diluted because there was enough push back. But not enough to ban “unusual” firearms that were “particularly concealable”. It’s a gangland America law. Keep in mind what was occurring at the time.
Context matters.
Makes no sense today. Kind of didn’t then, but they didn’t mind trampling some rights in an attempt to make it harder for gangsters.
I have read a little about the history of NFA. I do respectfully disagree with the inference that "gangland America" permits politicians the ability to violate rights of law abiding Americans. I suspect you don't agree with that inference and were just relaying what you believe the justification was then. However, politicians today would use "mass shootings" as their justification.
Based on what I have read about the roaring 20s and 30s, most gangsters stole their weapons from national guard armories and police stations. I have not researched to verify the accuracy of that claim but that is what I read. Regardless, when the ATF classifies the use of arm braces on pistols as SBRs, the scenario I gave should be the method of attacking it. Not even addressing the constitutional question, SBRs would not even pass rational basis then or today. Just my opinion.