Well, "they" didn't. A bunch of congressmen in office in 1934 did. And they had very specific reasons for it, too.
It all makes a lot more sense if you understand that the original intent of the NFA was to regulate out of private ownership ALL "concealable" weapons -- most specifically handguns. They included SBR/SBS "made from a..." language because it was understood that folks who couldn't buy handguns would just buy a rifle and make a concealable weapon out of it. So they wanted makeshift, improvised handguns made from other firearms taxed to the point of being effectively banned, as well.
Thankfully, handguns were struck out of the law before it was enacted, but all the stop-gap "made from a..." SBR/SBS stuff was left in the law more or less as an oversight, near as anyone can figure.
Since then, public opinion, which never really understood the matter to begin with, congealed around the idea that SBRs and SBSs were banned because they were somehow MORE dangerous than handguns or regular rifles and shotguns. It makes no sense and is of course utterly erroneous.
Unfortunately it is STILL federal law, and the ATF is left in the unenviable position of having to enforce these quite nonsensical laws as best as they can.
Even more unfortunately, since public opinion (and even opinion among most shooters!) has this ingrained idea that a "sawed-off shotgun" (or rifle) is extremely deadly dangerous (since, didn't they make them like TOTALLY against the law? ) trying to repeal the NFA is a terribly difficult issue and no law-maker wants to be seen as the guy who wants to let these KILLING MACHINES loose on our public streets.